for custom mods you need to load them into radiant via creating your own texture.pk3 (script and textures)
folders:
- mytextures.shader---------> scripts folder
mytextures.tga/.jpg*-------->textures folder
*you can save your custom textures as .jpg and keep the directory as .tga
i.e. textures/mystreetsign.tga could actually be mystreetsign.jpg (just leave .tga in the script, radiant and mohaa will still read it )
also remember, any custom textures you use will have to be included in your final map .pk3 file, you'll just have to make a texture folder for them.
grab a shader from pak0.3.pk3 (central_europe is a great one) and try and match up the texture you're making with an allready existing texture (i.e. if you're making something that would be made of road material, find a line in the shader that points to a mohaa road)