Right. Here's a list of how to do it. As Shadow says, you will need to have working HMT, so you need to fix that problem before you do this (check that you have .net framework 1.1 as this is the most common problem). Also, what do you mean by "I can't find my folders"? What folders? Anyway, here's how to do it, for those who don't know.
1) Download and unzip the two .tif files
2) Open the bitmap in
GIMP or PS if you have it.
3) Resize the textures so they are 512x512.
4) (only applies if you are using GIMP at this stage) Save the new 512x512 textures.
5) (only applies if you are using GIMP at this stage) Use
this program to convert the image to a .dds file. To do this, simply open it and then go save as .dds. In the dropdown menu select dds DXT1. Save it as a dds. I think PS can save .dds anyway. I didn't/don't use it so I don't know.
6) (only applies if you use PS) Save as a .dds DXT1
7) Open the map in HMT and go to bitmaps\weapons\assault rifle\fp\bitmaps\
8) The textures you want to replace are: assault rifle multipurpose and assault rifle_fp. You should then go "inject texture" and inject the relevant .dds
9) Close HMT and play halo. You're done.