i dont know how to active dlcs and other additional stuff in game

There should be an "Other Campaigns" option in the main menu. Also, double check the "Downloadable Content" option--which is directly below the "Other Campaigns" option--to make sure everything was installed.

in downloadble content its offering me to buy dlcs no matter that i have ultimate edition. moreover when i pressed buy the game said "unable to connect dragon age servers"

The "Downloadable Content" option was just to show you everything that was installed. Most of the dlc is already in the game and they will unlock as you progress your story.

The dlc that are in the "Other Campaigns" option are stand alone dlc and expansions. Pick one of those from the list and hit "Play" down at the bottom.

when you go to the Downloadable Content, it gives you 3 menus

make sure you select and click on Installed Content...

There it will show what is installed..