DarkSky International prefers online contributions through their web site. However, if you prefer not to do this, you can send a check to
DarkSky International
5049 East Broadway Blvd #105
Tucson, AZ 85711 U.S.A.
Make sure to include a note with/on your check about how you wish your contribution to be used, e.g., as a donation towards the NM chapter, for membership, etc.