Can't add new Time Capsule to existing wireless network

Hi,

I just got a 2TB Time Capsule to use as a wirless HD for Time Machine backups as well as manual backups of my photo collection. I have been trying for hours to get it to join my wireless network, but it just won't work. They guy in the store told me I could backup my laptop wirelessly, but it only seems to connect to my computer with an Ethernet cable, but even then it won't backup because Time Machine Cannot find the Disk. Now the only option it's giving me in Airport Utility is to Forget this Base Station and remove it from the network.

Please help!! What I want to do is have the TC in my office, connected to my wireless internet network WITHOUT plugging it into my wireless modem (which is in another room) so that I can wirelessly backup my hard drive. If possible, I would also like to be able to plug my printer into the TC's USB port, and print wirelessly as well.

If anyone knows how I can do this, please get back to me.

Cheers!