If you want to just test this mod, or there's any other reasons, you can do one of the 2 steps:

1. "PACKNHEAT" cheat. It will give you a default Postal 2 weapons, but to turn them into beta ones, you need to drop them and pick up again.

2. "MUTATE BETA" cheat. Please keep in mind that you will get ALL weapons that was made for this mod, even those that is not supposed to be picked up in normal gameplay (like Cat Launcher or AWP Grenade Launcher). This cheat is not requiring to type "SISSY' first. And hey, don't forget to activate "BMP: Beta Weapons Replacer"!