Ah. I've been here before, and the Venerable Mr PRR pointed out that not all computer/forum systems will cooperate, but none the less, here's how:
It's one of the (press + hold [alt]) [x], [x], [x] (release [alt]) sequence. (I use a windows 2000 at work, XP at home, PC computers...)
Ω = (press + hold [alt]) [2], [3], [4] (release [alt])
£ = (press + hold [alt]) [1], [5], [6] (release [alt])
± = (press + hold [alt]) [2], [4], [1] (release [alt])
√ = (press + hold [alt]) [2], [5], [1] (release [alt])
ⁿ = (press + hold [alt]) [2], [5], [2] (release [alt])
² = (press + hold [alt]) [2], [5], [3] (release [alt])
... Often times, as PRR observed (this was on the 'old' forum, before the big tree got blown down by the winds of change) this might display correctly in the "post text" window, but get mutilated in the final page assembly, but I find enough symbols in the list to be useful.
As a suggestion if it works for you and you like this method, spend a few minutes in "word" or something similar, and type a list of all the results that you get with various number combinations next to the 2 or 3-diit number that produces that character; then print it and tape it to the inside of a cabinet door near your monitor, or somewhere similar... it's what I did while I learned many of the most useful number combinations.
Keith