I assume the model editor is War3ModelEditor? If so, go to Material Manager (top bar -> Windows -> Material Manager) and it will open a list of materials. Double click one material and it should open another window with Material Layers. Double click one layer and it should open yet another window. In this window there is a "Texture ID" group (I have it in upper-right corner). In this group there is a list with textures (it looks like a simple one-line field, so you have to click on it). If the chosen texture in the list is the model's texture you removed, change it to your custom texture. Repeat this process for all MaterialLayers and Materials.
Do note: Since you already remove the original texture, I don't know if there will be any texture set in the MaterialLayer window or if it will be empty. Guess you'll have to try.
Also, your custom texture should be in specified path from the model. What I mean by that is that if you import your custom texture as "Textures\MyTexture.blp", then for it to work in the model editor you have to create a folder where your .mdl file is called "Textures" and place inside it the "MyTexture.blp" file.
