Ok I have a big dragon ina big room , I want him to just Walk around and not FLY .. how can I do that? the room is totally empty and has the navmesh on ground , no other items or markers are in the area , I just placed a dragonperch marker and a dragon ...

There is a spell in vanilla Skyrim called "AbNoFlightMG07Dragon". It's used to keep the undead dragon in Labyrinthian grounded; adding it to your dragon's spell list should keep it on the ground, too.

IMPORTANT EDIT: That existing spell also makes the dragon weaker to magical attacks. You may want to create a new Ability Spell that uses only the "ABDragonFlightless" Spell Effect and add that spell to your dragon.

added but doesnt work keeps flying .

so any ideas on how can I make it just walk and not fly?

http://www.creationkit.com/SetAllowFlying_-_Actor?