I cannot upload files via FTP

Huy guys of WPMU,

I can not upload files via FTP to my web server:

Estado:	Directory listing of "/web/MYWEB/public_html" successful
Estado:	Deleting "/web/MYWEB/public_html/backup.sql"
Comando:	DELE backup.sql
Respuesta:	550 Delete operation failed.

It seems that my user doesn't have permissions and my web hosting (Raiola Networks) doesn't helps if I don't pay... Could you please tell me with which SSH command can I give permissions to my FTP user?