In short the instructions are simple enough to follow.
first ssh into the RaspBMC terminal.
to do this either use Putty or use another linux machine and type
ssh pi@xxx.xxx.xxx.xxx
enter yes if needed
and the password is raspberry
wget http://xbmc-iplayerv2.googlecode.com/files/iPlayer-v2.4.15.zip
Then on the XBMC front screen navigate to System -> Setting -> Add-ons -> Install from zip file
now select the Home folder -> home -> pi -> iPlayer-v2.4.15.zip
You should now have the iPlayer installed as a video add-on. Works great for me 🙂