First you level up to the rank of Rear Admiral, then you go to the Pelia sector. I don't know. I suppose, if the transwarp gate lets through lower level players, you might be able to get into the fleet action at an earlier rank.
I still don't understand..... how do you get there?
There is a Transwarp Gate in the Sirius Sector Block (near the Eta Eridani Sector Block). That transwarps you to the Gamma Orionis Sector Block. From there, you can go the Pelia Sector Block, and a few missions there lead you to Fluidic Space. Note that Pelia itself is a "normal" space sector block, not fluidic space already. There is one fleet action that occurs in fluidic space.
But before you can use the Transwarp Gate, you have to be Rear Admiral 3 or higher.