Those "rubber things" are called grommets. They are used to isolate the servo from the structure of the plane in order to keep vibrations from the plane destroying the servo. The grommets are placed over the mounting lugs on the servo. Then you place the brass eyelets through the grommet from the bottom. THen you place the mounting screw in and stop as soon as the screw touchs the grommet. Don't tighten it down as tight as it goes or you will negate the effects of the grommet. See the attached diagram.

Ken