Antoine,
Adding your jar file to the ‘Append to classpath’ list on Window > Preferences > MyEclipse > Application Servers > Tomcat 5 > JDK should be all you need to do as this literally places your jar file at the end of the classpath that is used to launch Tomcat. Can you try this and tell us exactly what errors you’re getting when you do it, versus what happens when you don’t?
Also, can you please post all the information we request in the [URL=http://www.myeclipseide.com/PNphpBB2+file-viewtopic-t-393.html]Posting Guidelines[/URL] thread at the top of this forum? That will give us some context so that we can determine if this is an installation issue, a configuration problem, or a bug. Thanks.