It is confusing - and I've been doing this now for 15 years....
The best place to start, I think, is Fix Central. http://www-933.ibm.com/support/fixcentral/
The base code (9.0.1, in this case) is downloaded from Passport. The check Fix central for updates - http://www-933.ibm.com/support/fixcentral/ - then:
Product Selector - type 'Domino' (select IBM Domino from the pulldown)
Installed Version: 9.0.1
Then browse to the fixes, and download 9.0.1FP2
Your version is now 18.104.22.168 - so, start Fix Central over, starting with installed version 22.214.171.124, and you'll find IF1 for FP2.
See how easy that is?
Seriously - Fixpacks will create a 'Installed version change - in this case from 126.96.36.199 to 188.8.131.52.
Interim Fixes are a quick way to release code that don't increment the version number, and sometimes don't even have an installer - just a handful of files.