NOTE: Currently, if the VisualVM path is missing, starting a run of your code with the VisualVM launcher will show:
An internal error occurred during: "Launching [main class]". java.lang.NullPointerException
To fix this, go to: Properties->Run/Debug->Launching->VisualVM Configuration and fill in the "VisualVM Executable" box with the correct path.
The executable path will be
[path into folder where you extracted contents of your VisualVM download]\bin\visualvm.exe or
.../bin/visualvm, depending on your OS (as Sha commented).
If you cannot find this path in your system, it's likely because you've only installed the VisualVM Launcher, but not the VisualVM itself (as M.Hampton mentioned).
You can get VisualVM from here: http://visualvm.java.net/download.html#first_steps
(don't click the Eclipse IDE integration 'cause that's just gonna take you to the launcher again).