How do I turn a folder into a .jar file?
Jar files are almost zip files in most cases, but I would use the jar cf command for best results. 

http://docs.oracle.com/javase/tutorial/deployment/jar/build.html

You could just use OS X compression and then rename the zip file as .jar - but then you might not have a correct manifest file. It's hard to tell without the exact files in hand.