I am wanting to install a breaker box inside of a house addition and the box I bought has a 100 amp main breaker for cut off. I'm only going to be pulling around 4500 to 5000 watts at the most so I was wondering if I could run 10/3 UF wire and still use the 100 amp main breaker? I read on other forums a 30 amp main breaker could used.
For a subpanel application -- what you can do is pull the main breaker out of the panel and use a set of main lugs instead, as the panel disconnect is provided by the upstream feeder breaker (which will be a 30A unit given that your feeder is a 30A feeder). In particular, since the 100A MCB BR panels use a backfed type BR main breaker, you can simply pull the main breaker out and replace it with a BRSF125 main lug kit.
Of course, if you want a single disconnecting means at the subpanel, you can leave the main breaker in there and use it as a disconnect switch.
Breakers protect wires and receptacles. The wiring from the main panel to the subpanel must be protected by a breaker. It must be in the main panel, otherwise it can't protect the wire run! In other words: if the main-sub run is 10 AWG, it must be protected by a 30A breaker in the main panel.
The "main" breaker in the sub-panel is redundant/irrelevant and can be of any value. It is effectively just a local shut-off switch. You don't even need one, but you might get a better deal buying a panel/breaker combo intended to be a main panel. There's no difference, except ground and neutral must be isolated separately in a sub-panel.
If you don't have 2 spare spaces in the main panel, look for quadriplex breakers (possibly without the outer handle tie).
It's unfortunate you've already bought the 100A breaker, but NO. The breaker is there primarily to protect the wires, and only secondarily to protect the loads. A 100A breaker will not protect 10AWG wire adequately. Said differently, 10AWG is grossly undersized for 100A breaker.
You have two options: replace the 100A breaker with a 30A breaker and use the 10/3 UF cable, or keep the 100A breaker and install 3/3 UF instead. Installing 3/3 is no picnic (it's very stiff), and will likely cost more than a 30A breaker, but is definitely more "future proof" if you feel there will be more loads added to the subpanel (like electric floor heating, a stove, etc.)