You have to widen by grinding or filing (I use a dremel) the part of the safety that would contact the inside of the right side of the receiver. Take it slow and widen the gap ever so slightly until it will fold down.
First you may have to open the hole in the receiver slightly with a round file. Go slowly and keep checking the fit. It should NOT be sloppy or loose. You don't want it so tight that it binds, but you also don't want it to flop around.
Once the selector fits into the hole, then see if it will rotate down. If it does rotate down to the safe and back up to the fire positions, you're golden. If not, you may have to file the gap on the selector to get it to rotate. Once you install the selector, you will be able to see where it might bind up. Like Kevin said, the Dremel cutoff wheels are great for this.
To fit the selector to the trigger...
1. Install the selector lever and rotate it to the "fire" position.
2. Install your trigger. You don't need to install the hammer or disconnector yet.
3. Hold the trigger in the "at rest position". This is how the trigger would normally sit if the springs were holding it in place. The legs on the back of the trigger should be sitting flat on the receiver bottom.
4. While holding the trigger "at rest", start rotating the selector lever up towards the "safe" position. The tab will jam against the back of one of the trigger legs.
5. Scribe a line on the tab where the top of the trigger leg is. This will show you how much of the tab to grind off.
6. Remove the selector lever and grind off some of the tab, but stay a few thousandths shy of the line.
7. Reinstall the selector and trigger, and try the fit again. You will probably have to take a little more off, but work slowly and keep checking the fit.
Ideally, there should be very little play when the selector is in the "safe" position. The trigger won't move at all on "safe", if you fit the selector properly.