That plastic ring goes over the smaller diameter BBR60 cups to allow you to use the standard HTII installation tools and the one that came with mine seems plenty rugged enough. Go to Shimano's on-line dealer manual for installation details.

Here is a source for the plastic fixing bolt for only $3. You should really use it. The plastic "finger tight" installation tool TL-FC16) is nice to have but can be improvised.

Shimano FC6600 - FC6603 Crankarm Bolt | Chain Reaction Cycles

Shimano TL-FC16 Crankset Arm Tool > Accessories > Tools > Crank & Chainring Tools | Jenson USA

BTW, since this is a triple HTII crank, there should be a rubber two-part spacer that must be installed on the non-drive side between the nds crank arm and the bb cup. Be sure you have it. Again, the Shimano manual will give all the needed details. Here is a source for it too:

Shimano Hollowtech II 3mm Triple Crankarm Spacer | eBay