If you do not see the LayerManager button on the View ribbon tab, then please check that the add-in is installed by going to File \ Options and checking the Add-ins tab.
If the LayerManager add-in is not listed in the Active Application Add-ins list, then open the COM Add-ins dialog to tick the LayerManager on there:
If you do not see the full LayerManager functionality, then there are two possible causes:
The first is that you have not switched on Developer Mode from File / Options / Advanced.
Then restart Visio, and apply the activation code when asked, or from Layer Manager / About / i.
If required, VSTO can be downloaded from:
https://www.microsoft.com/en-us/download/details.aspx?id=48217