We have a mix of Win7 and Win8 computers, with ArcGis 10.2.1
On the Win7 computers, under Add-In Manager > Options tab, we can add a well-known folder to the box "Search for additional Add-Ins in these folders". This path is retained by ArcMap, (possibly in the Normal.mxt since it is present in a blank ArcMap doc, before opening an mxd).
This path is not retained on Win8 machines.
Anyone encountered and solved this?