Using input variables in the same cell

This is probably and easy one, but the Excel help files are anything but easy...

I want to multiply a user input value by another cell and have to total displayed in the input cell.

ie, I want to input in C1 the number "2". When the 2 is input I want it to automatically multiply that by A1 (which we'll say has a value of 3). The result being that cell C1 reads "6", even though I typed "2".

what does that mean? Or how do I do that? I am on a mac, so the ALT F11 does nothing. If I use the help file, it doesn't include VBA at all? There is not just a function that allows me to do this?