How do i get chargeblades with Castithan charge pack?

[Bentu]

Just to confirm that I need to start the main story missions again to access the arenas?

[Atticus Batman]

Just to confirm that I need to start the main story missions again to access the arenas?

I didn't have too. A mission icon should show up somewhere. If you did get the paid DLC, it is listed in the missions list (forgot what it's really called) under Castithan charge pack.

[Atticus Batman]

The mission you need to look for is called "whisperer's Scream" or something like that, around Raider's Landing somewhere. You will be collecting recorders for it. After that the next mission will be the arenas.