mongen.txt should have some stuff in it e.g:
;WommaTemple Entrance¸
D021 50 44 CaveBat 60 40 20
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveBat 60 10 20 100
D021 50 44 CaveMaggot 60 30 20
D021 50 44 CaveBat0 60 1 60
yours is probably empty.