You discover that the TFS 2015 Office add-in doesn't work because it's not signed by a trusted publisher. This issue occurs when the following conditions are true:
You're running a 32-bit version of Office on a 64-bit computer.
You have a version of TFS that's earlier than TFS 2015 Update 3.
In your Office settings, the Required Application Add-ins to be signed by a Trusted Publisher check box is selected under File -> Options -> Trust Center -> Options.
This issue occurs if there are quotation marks in the add-in's Location field. This prevents Office from verifying the publisher. To confirm that this is the cause of your problem, go to File -> Options -> Add-ins ->, select the Team Foundation Add-in, and then view the Location field to see whether the location path is inside quotation marks.
A fix for this issue is included in TFS 2015 Update 3 and later versions of TFS. Consider updating to Update 3 resolve this problem.
If you have an earlier build, you can delete the quotation marks from the applicable registry key. Make sure that you create a backup copy before you edit a registry key.
To do this, you can use a .reg script file or manually edit the string in Registry Editor. To manually edit the string, follow these steps:
Right-click the (Default) entry, click Modify, and then delete the quotation marks at the beginning and end of the value in the Data column.
If you still encounter errors after you delete the quotation marks, make sure that Microsoft Corporation is listed as a trusted publisher on your computer. If it's not, you must install the Microsoft certificate.