Ok, I assume you mount c to the physical c drive (or whatever letter your hard disk is), which means you did
mount c c:\
You then need to change to the directory containing CustLemm or LemEdit. On my home computer for example I put LemEdit and its variants at c:\lemedit, and custlemm and its variants at c:\lemedit\custlemm. So I would do
cd "c:\lemedit"
lemedit
to launch LemEdit, and
cd "c:\lemedit\custlemm"
custlemm
to launch CustLemm. Replace "lemedit" or "custlemm" with the filename of the various hacks (eg. nocdlem, custlem2, lemedit2 etc.) to run the corresponding program.
Note that because DOSBox emulates real DOS, filenames cannot be longer than 8 characters. If your directory path to lemedit or custlemm has a portion that is not named in a DOS compatible manner, I suggest you rename it. So no c:\thisistoofreakinglongforDOS\lemedit.
Hope this helps.
Oh yeah, almost forgot: for CustLemm and CustLem2, because it expects a CD-ROM drive, you need to also do
mount d d:\ -t cdrom
(replace d:\ with whatever drive letter your CD-ROM is on your computer) before CustLemm/CustLem2 will run. The hacks nocdlem and nocdlem2, as their names suggest, does not require you to mount the CDROM drive, as they play the original music that comes with the game.