Ah, I see, thanks for the link to the code. That's a pretty neat project!
@dxw00d is right, you'll need to reconcile the I/O ports and registers on the ATmega168 (used on the Pro Mini) with those on the ATmega32u4 (used on the LeoStick) which may be tricky.
For example, on line 558 of rx2atp.c there's a reference to UDR0, data register for the serial port on the ATmega168. The 32u4 uses UDR1 instead because of the built-in USB support, so you'll need to replace references in the source to "UDR0" with "UDR1".
That's just one of the changes you'll need to make, but if you can match up the 168 and 32u4 ports you should be able to work through the errors and fix them up.
You've selected quite a complex project to get started with if you're just beginning with Arduino!
Jon