Assuming you're downlading RSyntaxTextArea here:https://sourceforge.net/projects/rsyntaxtextarea/files/rsyntaxtextarea/2.0.1/
To include the source in your project, download and unzip rsyntaxtextarea_2.0.1_Source.zip. Assuming you're using Eclipse, both the src/ and i18n/ folders need to be included as Source Folders (alternatively, you can copy the contents of either, or both, into an existing Source Folder in your project).
Also, if you download and unzip rsyntaxtextarea_2.0.1.zip, you'll find rsyntaxtextarea.jar, which you can simply add to your build path. From your message I'm thinking you need help getting the source distribution imported, but I included this just in case.
Hope this helps!