Many people aren't aware of the differences between entering security codes when the router is in WEP mode vs. WPA. When entering security keys, if you see the 2nd field appear on the setup screen that shows 'key # (1-4)' then the router is using WEP and only hexidecimal security codes may be entered. You can use www.wepkey.com to covert a passphase to it's hexidecimal equivalant. With WPA you can use a passphrase and it's case sensitive. Other notes are below:
For SSIDs The following 6 characters are not allowed: ?, ", $, [, \, ] and the following 3 characters cannot be the first character: !, #, and ;.
Apple products automatically use the apostrophe character as a prefix, so the SSID must be manually edited to remove this.
For security keys:
For Apple products using WEP:
Find the security key by using the Airport utility.
Go to the "Base Station" menu and select "Equivalent Network Password."
For WPA passphrases, to enter words or phrases with the remote's keypad:
Triple tap the number keys, like texting on a cell phone (like T9 language settings on a cell phone).
Ex: Press 2 once for the number 2, twice for the letter A, three times for B, etc.
Keep pressing the same key to scroll through upper to lower case.
Press zero  for a space; press zero twice for 0.