Open the file by clicking on: Arena Editor.exe.
You will now see the interface (image below).
You can now start making your arena file, if your finished and you want to save your file click on: file/save as.. (image below)
Now name you file and save it.
If you want toload an arena file click on file/open (image below)
Now select the arena file you want to edit and click on open.
Here you can see an example of a RtCW arena file:
Here you can see an exmple of a ET arena file:
Here's a small explenation of what all the commands mean:
map: file name for example if you want to call a vote for it do: /callvote mp_test
longname: the full name
Type: gametype of your map for example Mp (multiplayer)
Timelimit: mission time in minutes
Axisrespawntime: set the respawn time of the Axis players
Alliedrespawntime: set the respawn time of the Allied players
Mapposition_x: X map position (ET only)
Mapposition_y: X map position (ET only)