This answer is pretty basic advice for how to access any binary conveniently. As with any binary on any system that supports terminal style inputs (e.g. bash shell or Windows Command Prompt windows), either the binary needs to be in a well-known location or you need to add a copy or a symlink to that binary in a well known location, or you need to provide the absolute path.
I created an executable script named "plantuml" to launch the jar file in command-line mode and put the executable script in my ~/bin directory, which is in my $PATH. I also put the JAR file in a stable location where I can conveniently run it by double-clicking on the JAR. The "plantuml" script I wrote is:
java -jar /Users/trickydick/Applications/Developer/UML/plantuml.jar
Or you could put a line in your .aliases file that says:
alias plantuml='java -jar /Users/trickydick/Applications/Developer/UML/plantuml.jar'
So on systems with UNIX style command interpreter shells (e.g. bash shell) either of these should work fine, for JAR files or whatever.