Garmin Cadence Sensor

Hit mode, settings, general, data fields, biking, choose the number of fields you want shown on the face of the 305 then hit enter.  This will highlight or make the field on the top or top left box, you can use the arrow keys on the right side to make whatever box you want to change the data field in.  Hit enter and when the choices appear, find cadence and hit enter, you can also have cadence avg or for the lap.

That should do it.  I like having the cadence feature too.