We’ve been asked on several occasions about support for ArcMap version 10.3.1 in the latest version of the Trimble Positions Desktop add-in (version 10.3.0.2). Initial testing indicates that upgraded installations will generally work (i.e., ArcMap was upgraded after Trimble Positions Desktop was installed) but that new installations on ArcMap 10.3.1 will not see a properly registered add-in.  There are 2 workarounds:

• Manually add a registry entry to mimic what the installer is supposed to be doing. This will preserve the proper uninstallation behavior of the app. The name of the new registry key should be:
  HKLM\SOFTWARE\ESRI\Desktop10.3\Settings\AddInFolders
  (or if x64, HKLM\SOFTWARE\Wow6432Node\ESRI\Desktop10.3\Settings\AddInFolders)
  …and then you’ll create a new string value that points to the path c:\program files\trimble\trimble positions desktop add-in\addins.
• Manually register the PositionsDesktopAddIn.esriAddIn by double-clicking the file in c:\program files\trimble\trimble positions desktop add-in\addins. This will copy the .esriAddIn to Esri’s staging area (c:\users\<user>\documents\arcgis\addins) and ArcMap will pick it up there. In this case, an uninstallation of Trimble Positions Desktop using the Control Panel will not remove it from ArcMap and you’d have to use the menu options within ArcMap to remove the add-in