Engine trouble
 

I have a 2003 Boxster and this morning it had an idle issue where it was surging about 300 rpm. A friend suggested getting some fuel injector cleaner. I did and the check engine light comes on now(would that have to do with the cleaner) the surge seems to have stopped but the car almost stalls when I start in first gear. I read about compressor in ac unit causing a problem like this but the ac is not on. I tried to turn it on and it does not work at all when the engine is on, when I later started the car the fans run before I start the engine but once it is running no fans or ac. Any ideas before I take it to the dealer?

sometimes the fuel cleaners can irritate and worse, damage the O2 sensors. I think it makes them get too hot or something...

Anyway, if it is not damaged, just confused, it will throw a code and run crappy.
I would unplug the battery for a few hours to clear the code, and this will also reset the Air fuel mapping. If it runs good, but then crappy later with CE light, drive it to autozone or something and they will read out the code to you. I bet it will say O2 sensors.

If this happens, before you run out and get new sensors, @ 100$ x 4, run the gas tank really low, fill up with gas, no additive, and reset the computer again.

you might get lucky.

A contrary view
 

is that the Pcodes tell why the CEL came on and until you know the Pcodes you are only guessing. Get them read. Your original idle problem could have caused the CEL as much as any additive related cause.

And you don't want to run the fuel tank down to anywhere near empty as that exposes the fuel pump to overheating, just down to 1/4 tank and refilling it will dilute the additive you dumped in before.