Sometimes the Edit » Paste or Command+V is not active on some registration fields in software. For instance to register applications like CrossOver. The same issue when trying to copy from such fields via Edit » Copy or Command+C to copy out the serial number to store it in a database or spreadsheet for a permanent record.
One thing to try is to Control+click over the field and then select Paste (or Copy) from the contextual menu. This will often work even when the main menu does not.
[crarko adds: This sort of behavior goes all the way back to the classic Mac OS days. Obviously software vendors don't want the serial numbers passed around but in any kind of a managed environment you'd like to keep a master list of the software assets you have, and that includes the registration keys. Having done this and knowing how big a pain it can be to keep a proper inventory I'm a bit sensitive to the issue. I publish this hint for as much as anything as a reminder to vendors that paranoia will only get you so far, and there is (almost) always a workaround anyway.]