They work fine if I try to open them in the "Custom Lemmings" (aka traditional Lemmix) style. Not so much if I try to open them in the NeoLemmix style.
This bug seems to happen for all traditional Lemmix levels that are in the Dirt style (Genesis or not) when attempted to open them in the NeoLemmix style. I'll look into what's causing this.
In the meantime, there is a workaround: After you load the level and it gives the error message, open the Level Properties dialog (the one where you set the title, lemming counts, etc), click the "..." next to the Style box, and choose the correct graphic set from the list (NeoLemmix -> Dirt (Genesis)). Once you do this, the level will show up properly.