CompilePal is what most people use just because it's extremely easy, automated, and can pack pretty much any asset you need.
Download version 27.19, not 27. I can verify that 27.16 also works well.
Just make sure you have the pack step enabled like here.
Once you have it selected and then compile, it should automatically detect all of the assets you're using and pack them into the BSP.
It should say at the end of the compile log how many custom assets it found and packed by asset type.
You can always check if things are packed by opening up your compiled BSP in GCFscape and looking through the folders in the BSP to see if the content is in there.