Like others have said, a level designer is not a modeller. In most studios, these are two different jobs. While a level designer is part of the "design" team and can sometimes work on game design as well, a modeller is more on the artistic side, and works with the rest of the artistic team on assets in general.
If the engine you work with requires to use Max or other modelling programs, you'll be taught the basics if you don't have them but in the same vein as you'd design your level with very basic brushes in Hammer. You'll have to learn how to create boxes and other low poly stuff to get the general idea across.
As a level designer, if you actually start detailing the map yourself, you're most of the time not even doing your job anymore.