This issue is fixed in CA PPM 15.4 and higher.
Note that it best practice to open MSP after installing the Addin to insure that the ribbon was updated correctly, which prevents the issue from happening.
Workaround: Either open MSP to make sure that the ribbon was updated correctly before exporting a project from CA PPM or click on OK on the error message, and then export the project from CA PPM again. The error message will only occur once. After the first time, it will not return