Can you clarify what's getting you stuck? Supports are applied when you hit the "Print" button or export as a CMB, and you can see what they will look like by entering Slice Preview in the upper right corner.

If you mean that you're having trouble getting the supports to be generated in such a way that they're easy to remove, then I'm not sure this is going to be possible for this part. Since the hole through the center is round, and there are two holes at a right angle, there's no orientation that doesn't put support material through the middle of at least one of them.