I have a 430EX ii speedlite which is controlled by the built-in transmitter in my 600D. I want to use the built-in flash as a fill light while using the off-camera speedlite as a key light. How can I achieve proper exposure with this setup? I have tried yesterday using my speedlite in ETTL mode, but images were over-exposed. Thanks in advance.
You don't want to have the Speedlite in E-TTL mode. This tells it it will be getting control signals from the hot shoe connector. To use this mode off-camera, you'd have to be using an off-camera shoe cord like Canon's OC-E3 or Syl Arena's OCF33. That, or an E-TTL capable radio trigger.
Instead, you should have the off-camera flash set to Slave mode, with the same communication channel set as on the camera. It should be in group A for that setup, since it's the primary. You'd set the on-camera flash to group B, then adjust the flash ratio to suit. You want more power to group A than group B to make B (the on-camera flash) the fill.
Sorry to fall back to an RTFM type of answer - but it really is the best way to find this out! :-) Read section 8 of the 600D manual on Wireless Flash usage (p.189 - and the bit you'll specifically want is on p.197), in conjunction with section 3 of the Speedlite 430 EX II manual (p.25). You should be able to work out from this how to have both flashes fire, with the 600D as master, but have them in separate flash firing groups to have them at different power settings.