The *.map exporter is an AddOn, you have to activate it 
So.. making a map using Blender. The best way to do this is going to be to split the construction of the map itself into three 'categories' or type of mesh Object as this helps you organise your thoughts and the process as you work and all of which can be made in Blender with varying degrees of fiddling.
- Detail brushes - what the players generally see inside the level
- Structural Objects - caulk hulls and material not generally seen by the player
- Models
Although Quake 4 et-al don't use "Detail" flagged brush volumes, you still build fiddly objects from collections of brushwork. If these type of items are placed in a separate layer you can better export and manage the scene. Same is true of your "Structural" objects. Again Q4 doesn't make the distinction between the surface types, but for you working in Blender it helps to place these 'block-out' objects on their own layer as well. Same goes for models.
As the tutorial linked to above mentions, you need to figure out before hand how you want to manage smaller objects, if they are to be models you'll need to export them to ASE (or MD5 if they move) as well as determining if you want to
export a large object with everything in place, or single models which you have to place in Radiant - both have 'pros and cons'.
With regards to the map itself... if you're going to export the visible structure as a model then you'll need to make a caulk-hull to contain it, and that also means you need to consider
'optimising' the mesh by cutting it up into segments to reduce overdraw and other related issues.
Your primary concern will be to
try and plan as much of your project before-hand, it'll save you a lot of time and frustration. The above will get you started. Everything you need to know
is on the site but it's not necessarily in one convenient location because of the way the site has grown over time so you'll need to do a bit of digging around. Hope that helps (good luck and welcome by the way).
