Depending on how you have your machine set up, you don't need to use the SYMBOL thing anyway.
If you have a numeric keypad on your keyboard, try this:
1. Turn on NumLock and leave it on.
2. Hold down the Alt key, and on the numeric keypad, type 0 2 5 2.
3. Let go of the Alt key.
You should see
ü appear.
With practice, you can get quite adept at that.
I have a little chart on the wall beside my PC with the relevant codes that I use.
There's an Excel sheet with the full set
on my webspace here
The only tricky bit is remembering it's Alt-0 plus the code (three digits) - and let go.