I've found my CD player keys and the menu on the launch page to change them, but I can't enter them
The buttons underneath to save or 'save and launch' are permanently greyed out. How can I enter them?

Edit the file nwncdkey.ini found in your nwn install folder. Use notepad to do this.

OK thank you I will try that
Err, hate to be a pain, but where exactly is the NWN install folder as I can not find it... :-(
Thirdly, when will I receive my serial number keys from GoG? It's been 48 hours now and I haven't had them through yet :-(

Typically the install folder is

C:\NeverwinterNights\NWN\

If you are using windows, you can use the search feature to find the file by name.

gog usually responds to cd key requests within 48 hours, but this is a weekend so might be monday before you hear anything (I'm just guessing tho as I know nothing of how they work on weekends).

Sadly it's not on mine. When installing the game, everything seemed to get shoved in this URL :
C:\Program Files (x86)\GalaxyClient\Games

I've been through it with a fine tooth comb, no sign of a NWN Installer file or folder...

Did the search feature turn up anything when searching for the file by name?