Make sure you:
1. Locate your boat on the surface of the sea, if you are not sure if your boat is on the surface press the "S" key (F1=Shortcut menu).
2. Have enough crew in the wanted torpedo compartment (=the room where the torpedoes are).
3. Move the torpedo with your mouse to an empty internal reserves torpedo slot (empty=there´s not an other torpedo already in it).
4. Check that it´s beeing loaded by holding the cursor above it (cursor=the thingie controlled by your mouse).
Hope it helps.