I am connecting a 1/4'OD polyethelene tubing to my refrig's icemaker. I'm using a brass compression nut with an insert. The tubing leaks where the tubing goes into the nut, Am I tightrening too tight?

Probably. Plastic ferrules would be a better choice.

A plastic tube with a compression nut and insert but you did not mention the ferrel, is there one?