SOLVED.
After updating from 2.4.3 to 2.5.6 I too also lost my import button. C:\ProgramData\Garmin\Trainers\GTN\FPLN was no longer working even though that's what is indicated on page 9 of the manual. Well somewhere along the line someone changed or a change was made to the directory and the file path has now changed. No wonder you're all frustrated, the manual is incorrect.
The correct file path should now be C:\ProgramData\Garmin\Trainers\Databases\FPLN and not in the GTN folder anymore. After 2 hours of painstakingly fiddling around with this and working with a buddy of mine he was able to figure out the NEW correct path. I hope this helps everyone out.