Fear not. I had the same problem with my mods. Took me a long time to figure out why they didn't work online. Smacks self)
When I figured it out, I couldn't believe how glaringly obvious the solution was.
The reason that the weapons don't show up on other computers is because it's part of a custom package. By this, I mean that the .u file (It'll be something like: ZarkAssaultRifle.u or Zark.u in your System folder) that contains the code for the weapon is not a standard part of UT. Therefore, there is no automatic provision for the computer to send information pertaining to it to other computers. You have to get UT to recognize this mutator and tell it to send its information to other computers.
OK, end of long drawn out explanation. Open up your UT folder and go to the System folder. Look for the file named UnrealTournament.ini. Open this up and scroll down to a place where it says:
[Engine.GameEngine]
ServerPackages=SoldierSkins
ServerPackages=CommandoSkins
ServerPackages=FCommandoSkins
ServerPackages=SGirlSkins
ServerPackages=BossSkins
ServerPackages=Botpack
Then add to the bottom:
ServerPackages=ZarkAssaultRifle (or whatever the name of the package is)
If there are custom skins, or textures, they should go here as well. If you install a big mod like U4E, you'll see lots of new stuff automatically added to the ServerPackages part of your .ini file.
Hope this solves your problem.