I did answer a similar post about one month ago now on this subject with a detailed series of steps. Unfortunately I can’t find the post now, I’ll keep looking but in the mean time if you put “FX account” into the search there are a few related topics that may help.
EDIT:
Here it is:
http://community.quickfile.co.uk/t/usd-to-gbp-posting-not-balancing/3103/