The sleeve should fit pretty tight into the plastic opening/adapter.
The sleeve will slide in from the front of the opening (like pushing it into the dash). There is a lip on the front of the sleeve that will catch and keep the sleeve from sliding all the way through the adapter.
At this point look at the back of the plastic adapter you have, there will be some triangular shaped tabs in the sleeve on the back side of the adapter.You will need to bend these triangular tabs out to keep the sleeve from sliding back out of the plastic adapter you have. If you cannot get to them from the backside of the plastic adapter, push them from the inside of the sleeve to the outside.
After the sleeve is mounted to your plastic adapter, insert the back of the radio into the front of the sleeve. Push in till it clicks.