First install the latest, stable version (currently 2.9.1) of the Open-Source AADL Tool Environment (OSATE) following the instructions at http://osate.org
The BLESS plugin can be installed into OSATE by Help -> Install Additional OSATE Components.
Check the box beside:
BLESS Editor and Proof Assistant
Click Finish. Read, then accept, the licenses. Finish, again.
OSATE will ask whether to restart. Do so.