I appologize, I didn't realize that another mod was also needed. You will need the prop snapping mod: http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=787611845 .
It will allow you to alter props Z position. With that enabled, you should be able to use move it in combination with PgUp.

Are you trying to do it in game, or in the asset editor?

Use the MoveIt tool (http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=766190099) .
It allows you to adjust the Z value of a prop with Page Up/Down.