Put in the second disc you recieved with ODST.
It will load what is essentially Halo 3.
On that you will have the ability to go into Matchmaking, Forge, Custom Games and Theatre.
These will have all of the Maps available.
You need to use this second disc to use the extra maps. It is essentially Halo 3 without the Campaign, meaning, if you want to play on the Forge or online, then use this disc.
If you want to play the campaign, then you need to insert and use your actual Halo 3 disc.
And incase you still don't understand, you won't be able to access the extra maps via your original Halo 3 disc.
If any of that's wrong, someone correct me on it, but I'm pretty sure that's it.