How do I enable access from one imac to the other.

I have inadvertently 'always denied' access from one imac on my network to another and now I need to. Can any one tell me how?

System Preferences - Sharing

Check off the item(s) you want to share and it'll tell you how to access it.

Still having trouble

I have already tried the system preferences option but it didn't work.
I think I need to elaborate. I am running osx 10.7.5 on one mac (I can not update this mac's operating system any further, may be because its to old). On the other mac I have just updated to Mountain Lion. Now I cant connect the old mac to the new but I can connect the new mac to the old.
A window pops up saying to Check the server name or IP address, and then try again.
Both machines have different IP addresses and when I try to change it to the same one it says that number is already in use.
I am a bit lost.

They are not suppose to have the same address - Two different computers cannot have the same IP address - that would be like your house and the house across the street from you having the same street address. How would anyone know which house to go to?

If you want to share stuff on your ML machine, then you need to go to Sharing preferences on the ML machine and set up what you want to share before your Lion machine will be able to access it.

(PS: I also changed the title of the thread to more accurately describe what you want to do. That is "enable sharing" - not "undeny access".)

Sorry about the address thing, doh.
I have both machines on the same file sharing options. The lion machine can see the ML machine on the network but will not allow me access.
Thank you.