Sounds like you got the mount, you need the bracket. It has four holes for mounting the compressor, and bolts to the block. The piece I am thinking you have is the rubber mount that bolts to the end of the bracket, and then to the underside of the chassis.
Also you'll need the P54 bracket, not the P30, and you'll need all the small parts that go with it to mount and adjust the compressor, and pulley.