Jacars2 is a free (as in beer) program designed to connect you to ACARSD servers around the world via the Internet. 
Jacars2 requires the Java runtime environment (JRE) and an internet connection. You are on your own for the connection, but get Java now.
Provided you have the JRE installed, simply unzip Jacars2 and run Jacars2.exe.
A discontinued copy of JACARS 2.0 ACARSD Client software can be donwloaded here.