To use James's version just load it into one of your custom Library, assuming you have one if not make one.
For example our office we load three libraries for each Project the ArchiCAD 14 Library, a Project specific Library and we have our own Custom Library.
As the object is a Label object once you have loaded the correct Library the Object will be available via the Label tool. Select say a slab or wall and tick the Label Element in the ID tab. In the 2d plan window select the label and click on the pop up menu where it says "Text" and from the list use the
Slab Elev JM10 label. Once selected you can then choose various option in the Label to say if you want to display a level to the soffit or top of the slab/wall.
My version of the object here...
dl.dropbox.com/u/399074/sites/trapdoor/Slab%20Elev%20JM10.gsm.zip
AC versions 3.41 to 25 (UKI Full 5005).
Using AC25 5005 UKI FULL
Mac OSX 10.15.7 (19G2021) Mac Pro-2013 32gbRam AMD FirePro D500 3072 MB graphics