The info_location is a brush based entity (you probably already know that

)
Here's what I do:Create a 1 grid high brush, textured in NULL and turn it into a info_location. Now position it underneath the map. After that scale it to the size of the area you want named and go to the properties of the info_location give it a name.
That's all you have to do.
Things to keep in mind:- Info_location create a 2d map of the names in your map (they can't overlap each other)
- If you have an area you can't cover with one info_location due to the shape of the area or overlapping issues. You are alowed to use more info_locations with the same name.