Minu,
Go to http://tomcat.apache.org, and download the latest 5.5 release in ZIP form. Then unzip it somewhere on your hard drive.
Now go to Window > Prefs > MyEclipse > Application Servers > Tomcat 5, and setup that information. Then go to the “JDK” sub-preference node, and setup a JDK (1.5 or later) to launch Tomcat with. Preferably JDK 1.5.0_08 or later.
Now when you go to deploy your project, you can choose between Resin or Tomcat. There is no need to do anything else.