Unlike in the standard Lemmix editor, the skillset has its own edit window in NeoLemmix Editor. You'll also notice that 16 skills are listed instead of just 8 - that's right, NeoLemmix has 8 new ones! These new ones are exclusively avialable in NeoLemmix; they are not supported in any other engine, including SuperLemmini.

Each skill has a checkbox next to it's skill name, a checkbox next to it's skill count, and an edit box to enter the skill count. Which engine is being used determines which of these can be used.

Lemmix - Only the skill counts themself are editable.
Lemmini - Only the skill counts themself are editable.
NeoLemmix - Everything is editable.
SuperLemmini - Only the skill counts and the checkbox next to those are editable.

The checkbox next to a skill's name determines whether or not that skill is in the level's skillset. This is NeoLemmix-exclusive, since other engines all only have a fixed set of 8 skills - Climber, Floater, Bomber, Blocker, Builder, Basher, Miner, Digger. In NeoLemmix, you can't have more than 8 different skills (but you can have less than 8; the remaining slots will show as empty spaces).

The checkbox next to a skill's quantity determines whether or not that skill is limited. If the checkbox is not checked, that skill has an infinite number of uses. This property can be set in both NeoLemmix and SuperLemmini.

Finally, the number input box determines how many of that skill are given. This can be edited in every engine.