To say 'thanks' or 'no, thanks', you can just use the adverb benigne. Whether it's an acceptance or a polite rejection depends on what you do when you say it. Benigne ades means 'Nice of you to come. Benigne dicis means 'Nice of you to say so', which is an appropriate way to accept a compliment. The source for this information is Speaking of Latin, by Norman J. Dewitt.
IM is short for instant message, a type of communications service that enables you to create a kind of private chat room with another individual in order to communicate in real time over the Internet, analagous to a telephone conversation but using text-based, not voice-based, communication.
Definitions: 1. Abbreviation for internal medicine; intramuscular. The definition information for IM is provided by Stedman's. You can search our medical dictionary here.