If you're talking about Amalfi's pbo decryptor it's pretty self explanatory actually...
just start the Unpbo.exe and then hit the 'open file' button.. that will let you browse your computer for the .pbo you want to open..
then once you have the file selected you just hit the decompress button
wherever your original .pbo file was on your computer... Pbo decryptor will place a folder with the same name as the .pbo containing all the goods
:toocool: