.dmg not recognized

Help! I installed a bunch of programs a few days ago, including stuff-it and i recently downloaded programs off the internet (skype & thunderbird). It downloaded the .dmg files to my desktop, but when i click on them to install the program, i get a warning message saying that it is not recognized. I've tried to "Open With" Stuff It Expander, but nothing...please help.

Thank you.

You shouldn't need StuffIt to open dmg files, OS X handles them on its own.

Have you tried downloading the files again? Also, what browser are you using?

I've tried to download them again and it's more than just one program, it's all .dmg files. I'm using safari. I didn't have this problem before, only since i installed programs over the weekend (Adobe suite & Office suite).

ok, so third times a charm. I re-downloaded Skype & Thunderbird and finally got it to open. But now i have another program that a friend e-mailed me that i cannot get to be recognized, and i've tried this one more that 3 times... any suggestions..
basically it's a zipped file that i un-zip and it becomes a regualar file, once i open the file, the .dmg is in the file and when i click on it, it is not recognized.