I’m confuzzled
 

Hello! I just got this RC Car awhile ago and when it died I had a charger for it, I use a Traxxas 8.4v 3000MAH NiMH Powercell Battery. And I tried to charge it with an ONYX110 Battery Charger, it has 1A 2A and 4A on the bottom and a little switch to move it from side to side. I don’t know what to set it to and I’m completely puzzled with what I’m supposed to do.
Can someone help?

For that battery you can set it to either 1a, 2a or 4a. That indicates how many amps it will charge the battery at. More amps = faster charge. Generally it's recommended to stick with 1a per 1000mah capacity. In your case the ideal charge rate would be 3a for that battery but since that doesn't seem to be an option you can either go with 2a or 4a. I would go with 4a.

Okay but one question if I set it to a 4a how long do I wait?

The charger should have some sort of full charge indication. Like some chargers the light will turn green when it's done or the light will turn off. It should say in the manual.

Charging your 3000mAh pack at a 4 amp rate will take approximately 40 minutes to charge completely if pack was "on empty".

With NiMh anything above 1C or 3A will be more likely to false peak and might not get a full charge... 2A will take longer but will get a better saturation cycle.