The button should already be there. On the "Tools" tab look for CAM Express (rightmost on the tab). This should launch with the current Solid Edge file.
Answer from John PEarson from Designfusion;
This usually happens automatically, but if not you have to manually register the nxcam_dll.
To do this close SE and open a command prompt window.
Change your directory to the NX main directory. For you that should be C:\Program Files\Siemens\NX 9.0\UGII.
Then type in the following command: regsvr32 nxcam_64.dll and hit enter.
You should get a message window saying that this was successful. Now open SE and see if the icon is active.