Hi Ali,
First of all, based on the documentations provided by GNURadio, it's better to use QT GUIs rather than Wx GUIs. It looks like in the upcoming versions, the are going to put Wx GUIs aside.
Anyway, if you are using GNURadio Companion, just double click on "Options" block which is by default on the top left corner of your canvas. In the popped up window, select the graphical interface you want by changing the "Generate Options" parameter.
Note that in a single file you only can use either QT or Wx. You cannot use them both together.
I hope it helps.