You may want to open the N Panel in the 3d View, go down to the Shading section, and set shading to Multitexture and turn Shadeless on. This sets the active texture for all of the polygons on your mesh to be the one you wanted to edit. Select all of the faces on the model with the A key, then if the UV/Image Editor switched to a blank image go back to the UV/Image Editor and switch back to the texture you wanted. Open up the texture in the UV/Image Editor and enter Edit Mode in the 3d View. The next step after all of this is done is to split the 3d View so that you have a 3d View and a UV/Image Editor. This can be done with the hotkey Alt+Z by default, or there's a dropdown menu with all the view modes in the bottom bar. You'll need to set the viewmode of your 3d viewport to Textured. In the top bar of the program there's a dropdown that might say 'Blender Render', 'Cycles Render', or 'Blender Game Engine.' If it's not on Blender Render, click on it and change it to Blender Render.

Once you've got the model reimported into Blender, the next step is to make sure that you're using Blender Render. This is probably not what you want to do if you're just modding the texture. When you use the Unwrap operator, it replaces the UVs of all the faces that you've selected with low-distortion, unique UVs.

For one thing, you need to reimport the game model so that it has its original UVs.

Blender paint on model