The tab for an installed add-in does not appear in Excel's ribbon, but the add-in is checked in the Add-Ins dialog.
A Microsoft Office Update for July 2016 causes Excel 2010, 2013, and 2016 to block files which were created on another computer, due to potential security concerns. The add-in will load successfully when it is installed, but when Excel is later restarted, it will not load the blocked file.
Note that this can occur for any third-party add-ins, not just Peltier Tech's.
There are two ways to deal with the issue. The long-term solution is to install the add-in from a Trusted Location. The short-term solution is to unblock the add-in Workbook. In most cases a user may have to take both actions.
For a number of reasons, it is advantageous to store the add-in workbook in the User Add-In Library directory:
Any add-in stored in the User Add-In Library directory will automatically appear in the Add-Ins dialog without having to install it first, eliminating the need to browse to find it during installation. Also, the User Add-In Library directory can be made into a Trusted Location using the following protocol. Once the User Add-In Library is a Trusted Location, any add-ins can be installed from this location without having to be unblocked.
For some users, making the add-in's parent folder a Trusted Location has not been sufficient, and the add-in file itself must also be unblocked: see below.
This allows you to install the add-in from any directory on your computer. However, if you receive an updated add-in workbook, you must unblock the updated workbook before it will open.