kobih,
There are 3 requirements for successful JSP debugging:
1) JDK 1.4+ to run Eclipse and Application Server
2) Application Server supporting JSR-45 (Tomcat 5)
3) Start the application server from MyEclipse
The reason step #3 is important is because MyEclipse will hook the JSP source locater into the debugging process so breakpoints in JSP pages can be mapped from the Java class file back to the JSP page.