This is very strange. If I just open your file in my English version, the item is there (N01), but if I change my preferences to French, it’s gone. This is a programming problem - probably with the translations - I tried all languages and it seems it’s only the French that this happens. I will register a ticket for it to be repaired.
In the meantime, I suggest you change your preferences to English so that you can add the measurement and then change back to French, if you wish. It should still use the measurement correctly in Seamly2D.
To change your language preferences, click on File>Preferences. The option is at the top of the screen. Then click on OK to accept the change.