Once you have started up YAPE goto the option menu "FILE" and select "LOAD PRG". Find and add fire_ant.prg
Once you return back to the Plus4 screen type:
RUN
and the game will start. Check out the other menu options for sound, joystick support and loads of other cool things.
There are a good number of C16/plus4 emulators available some you can find on my homepage, but please as always check out the authors homepages.