Keyboards & HID

Some of you may remember a PS/2 keyboard I had some questions about that didn't work. I opened it up and it looks the mux circuit is shot. So I going to try to do it manually. (It's very complicated, but it is along the lines of getting a new mux for the keyboard and plugging that into the Arduino) I think it'll work. My real question is, how can I emulate this as an HID? Can I? I have a mac. (For starters, if its possible, could someone show me how to use a tactile button as LMB?)

I emulate this as an HID? Can I?

it was told taht it could be done with an uno but i haven't seen something like that until now

I saw some things about reprogramming (reburn a new bootloader) the Atmel, but I don't really want to.

What is an "LMB"?

Left Mouse Button.