KM,
Not sure how else to state this other than you are confusing a WTP construct with MyEclipse.
There are no “Server Runtimes” in MyEclipse, that is from WTP. There are “Server Connectors” in MyEclipse, and they are accessible from Window > Prefs > MyEclipse > App Server. These connectors cannot be added to any build path.
The Build Path > Library > Server Runtime entry is contributed by the WTP plugin set, not MyEclipse.