When u look at the things that are in the file.
There u see these joints, Bone01 Bone02 etc
These joints tell where the model can animate.
Open a different weapon
Click Character -> Weight Tool
And when that is opened click on the different bones u see.
U will notice that the model on the screen turns black and every bone u click another part of the model turns white.
Now u can just go to the model and start painting it white where u want it to animate.
There are examples enough on where to paint the weights on each model as every mesh has it. It can be a bit tricky at first but u will get the hang of it after a while