Hello Painteater19 -
Since the add-in in the version you are using is not for NX 7.5, it is best to uninstall it.
The other issue is one that happens in Win7 and Vista with the UAC blocking the command. If you want to temporarily turn off the UAC you can type UAC in the Start search box and lower the UAC slider.
The reason this is not a recommended solution is that it lowers the level of security. You may want to try it to verify that once 3DxNX is uninstalled the command functions are allowed to work in NX.