When i first started using logos, the log said it was working but I hadn't created the directory. Try creating a thumbs folder in that directory and redo the import. Be sure Media Portal is completely closed when you create the folder.
If that still doesn't work, try reinstalling Media Portal to see if it will create the folder with the proper references for you (if there is a repair option during installation try that). Beyond that... it's a Media Portal issue and not a MyTV Series issue I think. If you create the directory though it should be okay. Be sure to also check the 'About' tab in the MyTV Series config as the other folder directories will be listed there and you may need to create those as well if they aren't already.
If that still doesn't work, try reinstalling Media Portal to see if it will create the folder with the proper references for you (if there is a repair option during installation try that). Beyond that... it's a Media Portal issue and not a MyTV Series issue I think. If you create the directory though it should be okay. Be sure to also check the 'About' tab in the MyTV Series config as the other folder directories will be listed there and you may need to create those as well if they aren't already.