Essentially, you open it and choose prefered decoders.
On the 32bit side, just change from MS to FFDshow (Just be sure FFDshow is set to decode the ones you set it for)
You do not need the 64bit side, unless using WMC.
And remember to change them back to MS if you decide not to go this route.
You probablly already know, but for the record, this method is really meant for FFDshow+Haali Splitter type set up. Not really a full codec pack as some include similar methods in them.