Hi guys!
I thought I was gonna get crazy, thinking Pixelmator could not join two paths. However it all boils down to an understandable translation to Spanish error.
In the image above you can see the Spanish translations for the pen contextual menu (still called 'bolígrafo' instead of 'estilográfica' or 'pluma', but…). The option I needed to select was 'Acceder.' However, although 'acceder' can mean 'join' (as in 'joining a club'), the menu should read 'Unir' (join, unite, connect).
All the suggestions from my older post were usability-related, but no Spanish speaker will read 'Acceder' and think 'Oh, yeah, this is the option for joining these two points,' so I'd strongly advise you to change it to 'Unir' (or 'Fusionar' –to fuse– or 'Combinar' –to combine–) because, as it is, it's completely obfuscating the option.
Thanks for your work!