You should be able to use either a 6" or an 8" dado set on the 4512 with no problems.
What you describe as "release switch" is actually the release lever for the riving/kerf knife. You will need to remove that, along with blade guard and anti-kicknack pawls, before you install your dado stack. In order to remove the riving/kerf knife, just set the lever to the up position, tilt the knife to the right and pull it all the way up. Leave the release lever in the up position and install your dado stack.
Two things to remember:
1) You won't be able to use your factory throat plate with a dado set. I recommend using one. You can make your own (which is what I did), or order one from Ridgid.
2) Max dado depth on the 4512 is 1 1/2". If you try to set it any deeper than that, the riving/kerf lever will push through the throat plate.
See page 43 of your owner's manual for all the details. If you don't have the manual, let me know. I can email you a scan of the relevant page in the manual.