To decompile the models, you need to download an external program other than HLMV. The model viewer only offer a way to set up the decompile program for easy decompiling without command lines.
But you don't need to decompile a model to change it's textures with HLMV. Simply export the texture from model viewer, change it up, make sure it's 256-color indexed, and import it back in and save the model! If you're having troubles with that, it's most likely just a problem with the edited texture, and not the program. Make sure it have the same height and width as the original texture, and make sure it's an 8-bit image (indexed with 256-colors).