Create a level.nxmi file inside your rank folder. You can set the order of your levels there like this:
LEVEL level 1.nxlv
LEVEL level 2.nxlv
...
LEVEL level 30.nxlv
I recommend using an existing pack as a blueprint.
Just download any NL level pack, copy and paste its level.nxmi into your directory and edit it to fit your pack.
(You can edit level.nxmi with any basic text editor)
LEVEL level 1.nxlv
LEVEL level 2.nxlv
...
LEVEL level 30.nxlv
I recommend using an existing pack as a blueprint.
Just download any NL level pack, copy and paste its level.nxmi into your directory and edit it to fit your pack.
(You can edit level.nxmi with any basic text editor)

It's pretty obvious to me now that your levels are right up my alley. I may come back to play the rest of the pack once I finish playing the level packs I'm currently working on. Here are some comments on the individual levels:
It's so gruesome and since this level requires the compression method to solve, you have to sacrifice at least one lemming to that trap! Genius level design to showcase the new trap...
