Lumpmod closes the instant you open it because it's a command line program. In order to use it, you open a DOS prompt, navigate to the SRB2 directory, and type "lumpmod filename.wad add MAP01D filename.txt" where filename.wad is your WAD file and filename.txt is a plain text file with the level header information everyone is showing on here.
The level itself is pretty average. For a first try, it's amazing, however. The entire thing could use to be a little wider. A general guideline is that you don't want a passage skinnier than about 256 fracunits across, which is 4 squares in WadAuthor's default grid. Otherwise quite solid, and very nice for a first attempt at wadding.