Hi TheForge,
the seal on the bearings is usually already installed on the OSBB bearings. On a BB30 bottom bracket you need to put the seal on the bearings. A simple way to check if everything is in order is when you have removed the existing cranks and have the bearings in front of you whether they are covered by a rubber or plastic seal or whether you have the bearings right in front of you.
The Rotor 3D+ doesn’t come with bearings - you either get a BB30, PF30, BBRight or OSBB bottom bracket with it.
I hope this helps,
Best
Nicolas