Lost 3G connection in MacBook

Apologies if this isn't the right forum, but I don't know if it's a hardware problem or a software one until it's solved.
I'm trying to help a friend, who's completely new to computers and recently bought (new) a MacBook. There's no further model name on it; it's glossy white with rounded edges, and looks like the 'Mid-2010' one reviewed in a magazine I have (which probably went to press in August).

It connected to the internet in her home, using a USB dongle and a mobile-phone/cellphone 3G network (the only broadband available in her area). She then had a stay elsewhere, where she was able to connect by simply plugging in her host's broadband cable (DSL I think). But once back home, she couldn't connect again with the 3G dongle. The dongle shows between 3 & 4 curved bars (signal strength, we guess), just as it did when she was able to connect.
The only suggestion I could think of was to try the dongle in both USB ports, and both before & after booting the machine (that made no difference). She's now seeing the error message 'The server can't open the page because the server where this page is located isn't responding', so I guess things are at least reaching the handshaking stage.

Could it be something really simple, like having to re-enter a password? (I don't think she even knows to try that, being so new to computers; let alone how to find the setting).
I'm sorry I don't know any more details yet. I've been using (and tinkering with) computers since the 1980s (including many an internet connection saga), but haven't yet used any Mac.

My friend's had just 1 brief opportunity to come over here with the MacBook (we live 1/2 hour or so apart). I tried to find the kind of settings I'd check in Windows, but didn't know how to find them in the Mac.
I did find a screen with a choice of several methods of connecting to the internet, with Airport selected; is that correct for the 3G dongle method? Airport seems to be for finding wireless connections, but I don't know if its definition of 'wireless' includes a device that accesses the internet through the air but talks to the computer via USB? I couldn't recognise any reference there to this type of connection. Could the MacBook have recognised the DSL connection, changed any settings accordingly, and then not changed them back when the dongle was plugged in?
I couldn't try things with the 3G dongle receiving, as there's no coverage at my place (Wimax is the only broadband available here, with the phone line not even good enough for dialup).

I'd really appreciate any pointers you could give, or any relevant URLs. I've tried searching these forums and the Knowledge Base, but can't seem to get the keywords right. That may be because of not knowing the Mac terminology. (In case it saves people explaining it in any replies, I'm sure I can easily look up any terms I don't know).

I don't know where this forum is based, but I'm in Ireland. I have to end this session soon, but should be able to return here tomorrow (in about 15 hours).

Welcome, this is an international forum, you'll find plenty of people here that are also in your time-zone.

With the 3g connections, you basically 'dial' to the ISP or Cell provider, much like the old internet connections.

Airport would not cover the 3g connection that you are looking for.

If you click on Apple in top left and choose System Preferences
Click Network

you should see something like my attached image.

The 3g connection should be in that list.

In my attached image, I am currently using a home wifi link (Airport) but I also have 3g connections called Vodafone..., ZTEUSB... o2..., DIAG. I imagine your friend will only have one. Basically, it will be the one with the telephone handset to the right of it.

The best thing (in my mind) is to select your 3g connection, click on a tick-box towards the bottom of the preferences window (Show modem status in menu bar) as this then puts a little 'handset' logo in the top right notification area of the Apple Menu bar.

Then when you want to go online, just click on the handset and choose connect name of connection here.

I hope that helps.

First, these forums are based in the US.

What has probably happened, is when she temporarily connected via the ethernet cable (broadband DSL) the machine is now setup for that type of connection. She will have to manually go into her System Preferences, Network, and change it back to Airport.

It's probably best for her to contact Apple support there in the UK or Ireland and have them walk her through it on the phone.

Thank you for taking the trouble to go through it for me, RNDDave and chscag, and I must apologise for not replying. I thought I'd be reporting back soon; we arranged that my friend would ring me from her home (where there's 3G coverage), and I'd try and talk her through the process. She hasn't yet, for some reason, but I'll let you know what happens!
Thanks again.