mhliu,
To add JVM arguments to a server in MyEclipse 2015, you need to follow these steps :
1) Open ‘Server view’ and double click on the server
2) Click on “Open launch configuration” link.
3) Go to ‘Arguments Tab’ and add the JVM settings under ‘VM arguments’ section. (For your visual reference please take a look at the attached screenshot)
Hope this helps and let us know if you see any issues in MyEclipse.