Note again that here it's resident-guest replying to a passerby-guest:
I can't get CustLemm to run at all. When I try it with or without DOSBOX it displays:
"Open Has Failed: (some weird characters)
Exiting Lemmings..."
You're missing some files or the files are corrupted.
To run CustLemm you need pretty much all the other files of Lemmings. I believe Insane Steve or someone had some time ago uploaded a version of CustLemm that has all the necessary files from the original Lemmings, but I don't remember the URL to it.