I have played through the Purity Quest and endgame, but cannot figure out how to access the Expansion content in the PC Gome of the Year edition. Any help is appreciated!

Did you activate them under the Data Files option on the launcher?

Not sure- when I open that item all I see is a checkbox beside Fallout3.esm. Down below it says "load loose files" and it doesn't seem to matter whether that is checked or not. Shoulld the exp packs appear here-I dont even see evidence of installation files for them them on the install disks.

Check all the boxes, otherwise they wont run. Just check the boxes, then start playing as usual.