Welcome to the forum, @miket!
This might be caused by a recent change in how ImageJ 1.x handles csv files. From the release notes:
- 1.51m 30 April 2017
- Changed the default extension when saving tables from ".xls" to ".csv" (coma-separated values) and added support for opening and saving tables in ".tsv" (tab-delimited values) format.
You should be able to work around the issue by either:
- opening the file in the script editor (if you use the Fiji distribution, the currently recommended way of using ImageJ), or
- renaming the file(s) to
.txt and then opening them.
I don't think the original authors are active on this forum, so I invited Anja Hartmann to this forum topic, as they might want to update their plugin documentation.