If you haven't already, download Eyeball from the Eyeball package page and unzip it into a directory of your choice. The download is rather large, because Eyeball 2.3 includes the Jena jars so that it runs stand-alone.
If you have Ant installed, run the Eyeball test suite:
ant test
If it doesn't say that the tests passed, let us know via the jena-dev mailing list jena-dev@groups.yahoo.com (registration required) and we'll see what we can do to fix the problem.
Ensure all the jars in the Eyeball lib directory are on your classpath. (You might find this list helpful.)
Pick one of your RDF files; we'll call it FOO for now. Run the command-line command:
java jena.eyeball -check FOO
You will likely get a whole bunch of messages about your RDF. The messages are supposed to be self-explanatory, so you may be able to go ahead and fix some problems straight away. If you get a Java error about NoClassDefFoundError, you've forgotten to set the classpath up or use the -cp myClassPath option to Java.
(You may also want to try the experimental Eyeball GUI, but you'll still need to start it from the command line.)
If the messages aren't self-explanatory, or you want more details, you'll want to move on to the brief guide.