Sometimes my synced folders get deselected, I don’t know why. When this happens I just need to open the configuration and select them again.
EDIT:
Mega provides arch packages directly. you can add it to your /etc/pacman.conf
[...]
###REPO for MEGA###
[DEB_Arch_Extra]
SigLevel = Optional TrustAll
Server = https://mega.nz/linux/MEGAsync/Arch_Extra/$arch
###END REPO for MEGA##
You probably need to add the signature to the keyring first.