[Rey_a]

Comme le logiciel ne gère que
128 registrations dès que le multiple à atteint les 500 on est bloqué, car le passage au second groupe de 500 ne se fait pas

Claude,

In the above sentence, you describe two limitations of the system when configuring the system to load registration files on Tyros when loading a song in MobileSheets.
1) Without using the MSB in the patch select, there is indeed a limit of 128 different registration files that can be selected. But als you also describe further in your post, changing the MSB to 1,2, ... will remove that limitation giving access to much more files. 
As for you question where to add that, right click the song, select 'edit song', go to the MIDI tap, click the '+' at the right. see image below

   

2) The second limitation comes from the fact that only 500 files can be stored in a folder on Tyros. The patch select, can only be used to select a file from the 'current' folder on Tyros. There is no patch command to have Tyros change folders.
So if you need to change folders, the only thing you can do is to manually (on Tyros) load a registration file from that other folder. Then when receiving further patch selects from MobileSheets, Tyros will select files in that other folder (where it's currently loaded registration file is in).

Rudy