I was able to get around the error by copy the file HistogramPlus-S.exe from this location (Win7 using MS Office Professional 2010 32-bit):
E:\Users\Paul\AppData\Roaming\Microsoft\AddIns\HistogramPlus-S\
to
E:\Users\Paul\AppData\Roaming\Microsoft\AddIns\HistogramPlus\
and then renamed HistogramPlus-S.exe to HistogramPlus.exe (removed -S from filename).
Seems to work ok. Note that the add-in does add conditional formatting to the original data cells. Recommend to copy your data to a new spreadsheet and do some tests runs.
Hope this helps some users and perhaps the company supplying the addin.