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
enter yes if needed
and the password is raspberry
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