On my number pad, on the far, far right, in fact the last keys on the keyboard, going right, there's a big "Enter" key (well, the key is big, but "Enter" is written small). Immediatly above that, is a really big "+" key (Same as the "Enter" key, this one is big, but the "+" is small). Immdediatly above that, is a regular sized "-" key. Is yours configured the same way? Because pushing the + and - keys from anywhere else on your keyboard is not going to produce the results you're looking for, if it produces any at all. In other words, if you can't find a "+" and a "-" key on your number pad, take a picture of your number pad, and post it here. Are you on a laptop? If so, then it's possible that your number pad has had keys removed to save space.
The keys will increase, or decrease the emulated Dreamcast CPU. Those green numbers AND letters you're refering to should indicate which numbers represent the CPU speed. If you're not seeing those numbers change, then you're pushing the wrong buttons, or don't have numlock on. Hopefully I've explained this throuroughly enough for you. Good luck on getting it working. Mouser X over and out.