Grim,
If you are using the EJB3 annotations you need to make sure your project is a JEE 5 project *and* has the JEE 5 Library Set added to your build path in order for those annotations to be resolved.
We added JEE 5 support in MyEclipse 5.5, so the only way for those annotations to work with MyEclipse 5.1.1 is for you to create your own User Library with the JEE 5 libraries and add it to your build path manually. Is this what you have done?
If not, this is why the annotations are not working.