From time to time, the Excel process on my machine will remain in memory for a long time after I close Excel. This will cause Excel add-ins like Smart View and Cube Designer from loading properly.
The culprit is a corrupted Excel15.xlb file that has grown to be a large file size. On my machine, this file was ~10 MB. The normal file size is ~10 KB.
Simply go to this location on your Windows machine and rename the file:
Next time you start Excel, this file will be rebuilt automatically.