I am not a Mac user, but to transfer files from a Mac to the 550 the Mac must see the 550 as "Mass Storage" like an HDD or SD-Card.
this should usually happen when you confirm "import Routes" on the 550.
On a windows PC now the internal Memory and the attached SD-Card appears as a drive.
For some reason, the Mac can not see the 550 as "Mass Storage".
The only option, I guess, would be to use the SD-Card to get it to the 550.
The TomTom help page refers also to Android File Transfer.