Create a new plugin and choose “from existing archive file”.
If you want to add the JAR file to an existing project, the simplest way is to import the JAR file using “File -> Import …”. You now need to tell Eclipse that it’s something where Eclipse is allowed to read executable code from, i.e. you need to add it to the Java classpath. The simplest way would be to right-click the file in package explorer and add it to the build path. The problem is that the effect gets lost when choosing “PDE-Tools > Update classpath …”. Therefore, better follow the way described in this post.