The dielectric plate slides into the feed horn until it reaches the end of the groove. The tape can be used to keep the plate from sliding out or use a spot of silicone sealant on the plate's edge to prevent it from easily sliding forward.
Only insert the dielectric plate if you wish to receive circular polarity C-band signals (I.E. satellite NSS-806). Do not insert the dielectric plate if you will be receiving linear C-band signals as it attenuates linear polarized signals when inserted.