I think you should read this chapter
of Wakana's tutorial. It explains how ADSR works. It can be used with #instruments (which is a method that I'm more familiar with) or as a hex command in the MML (which the tutorial explains). The tutorial does explain the method with #instruments briefly as well.
For the ADSR thread, the "use with" portion is just an additional hex command that may be included with the custom instrument to further improve the quality, such as vibrato or amplify. Oh, and these commands can also be found by reading the AddMusicK readme.