There are a couple of things you can check. What does the transcript say in the output window? Any clues in there. Does the Symbol Editor application appear in the Task Manager? Does a process nse.exe show in processes. If so then the symbol editor could be outside of the screen area. Finally check you have write access to the library files (Operating System write access not DxDesigner writeable library access).
If none of these solve the problem contact your local Customer Support organisation.
I have the same problem. The symbol editor do not appear. In the output window appears:
Test_Block.1 - symbol definition updated
Started Y:\7.9.4EE\SDD_HOME\nse\win32\nse.exe -prj "H:\14\130902\A\IC_DSE_LogicBoard.prj" -parent dx -icdb "H:\14\130902\A\database" -snapshot "DxD" "Test_Block.1"
Seems that the editor is started and at once closed.
There is an issue if your Symbol Editor window has been adjusted in size (the default is maximized) - it is causing the behavior you are describing but only for Local Symbols.
TechNote mg580755 details the issue and a solution.