Thanks for the help but this did not work.
I am using the free version of tubepress 3.1.6 with wordpress 4.1.1.
I have continued to try various combinations of Flexible Thumbnail version and location installs with the following results. Firstly I should say that Tubepress did not create the tubepress-content subdir - I had to create that and the add-ons subdir manually.
Install flexible-thumbnail-rows_4.0.0 to www.mywebsite/wp-content/tubepress-content/add-ons/
Flexible thumbnail rows option does not appear in Tubepress thumbnail options. No change to thumbnail layout on website.
Install flexible-thumbnail-rows_1.0.0 to www.mywebsite/wp-content/tubepress-content/add-ons/
Flexible thumbnail rows option DOES appear in Tubepress thumbnail options. No change to thumbnail layout on website.
I also tried both add-on versions in www.mywebsite/wp-content/plugins/tubepress/tubepress-content/add-ons/
Flexible thumbnail rows option does not appear in Tubepress thumbnail options. No change to thumbnail layout on website.
For each attempt I tried enabling/disabling Tubepress in between installs/uninstalls, and using your clear cash method which just seemed to return a "page cannot be displayed" error.
Any further insight / suggestions greatly appreciated. Let me know if there is any other info I can provide to help resolve.