I'm not sure why it would not work in letters, but to get the spell ID number a simple way is as follows:
- Make a macro with the spells you need
- Give the macro a name you will recognize
- Drag the macro to a hotbar
- type /savehotkeys <filename.txt>
- open <filename.txt> in your eq2 folder and find your macro name
- below the line with the macro name you'll see one /useability <spellID> line per spell you added to the macro, copy that to your mod