If you have the Rise of Isengard expansion, you will be able to play in the Ettenmoors (with some restrictions).
From here:
The other big change in Rise of Isengard is PvMP. For the first time,
F2P (free to play) gamers can play in the Ettenmoors as a reaver (the
other monster classes cost Turbine points). Also, every monster class
skill is available for purchase regardless of rank, so the Ettenmoors
should see a large influx of new players testing it out. A recent
patch boosted monster players up with 40% more out of combat morale
regen and an across-the-board 20-30% DPS increase, making them much
more competitive against the free peoples.
Also, more information, from here:
Before you are able to play Monster Play, you have to create a free
player and attain at least level 10. After that, a red "Monster Play"
button will be active on the character select screen after login
(Prior to Book 10, Fell Scrying Pools were used in-game; these have
been removed). In Monster Play, you have one slot for each class
available to play. Free players get only the Orc Reaver as free class,
the other classes cost 800 Turbine Points. The available classes are:
Orc Reaver, Orc Defiler, Spider Weaver, Uruk Blackarrow, Uruk
Warleader, or Warg Stalker (all of which start as level 75 monsters).
Once created, each monster character will persist and be available for
further play, unless you choose to reset it.
