For users of Excel 4 (and a PowerPC Macintosh), I provide two "accelerated resources" containing the two interpolation functions. I do not provide Add-In support. You will need to add a macro to your sheet that does the following:
This macro will register the interpolation functions manually.
(If you are concerned about speed, you should get Excel 5 for Power Macintosh. The accelerated resource will help, but its rather pointless. OMDI does not officially support 68K Macintosh products.)