A computer is holding open a connection which is stopping other computers from working.. or you have loaded the sparse bundle in finder and it is not being unmounted.
Go to the airport utility.. (5.6 if Lion) and manual setup.. disks page.. disconnect all users.. you will then be able to use the device.
I am assuming Time Machine means Time Capsule.. it is a common mistake.
There are a number of other issues that cause the problem but that is the most common.
Some more info here.
Thanks for your help, and sorry for the mistake about Time Machine. As you can tell, I'm new at this. I have Mac OS X 10.7.3 and Airport Utility 6. I believe the OS is Lion, so I'm up to date on everything. However, when I open Airport Utility, I don't see the disk icon that is shown in the article you provided.
As I said you need to GET 5.6 utility.. the v6 utility in Lion has no disk interaction section.. how Apple how can you setup TC without disks.. ???
Google is always your friend btw... just look up ..
but here is the link.. http://support.apple.com/kb/DL1482
Download and install 5.6 utility and then you will have disk.