You can't use the backup and restore when using RDS to transfer data into RDS. You'll need to use another means to get the data in there.
If you have an existing Microsoft SQL Server deployment that you want
to move to Amazon RDS, the complexity of your task depends on the size
of your database and the types of database objects that you are
transferring. For example, a database that contains data sets on the
order of gigabytes, along with stored procedures and triggers, is
going to be more complicated than a simple database with only a few
megabytes of test data and no triggers or stored procedures.
RDS for SQL Server service does not currently support RESTORE DATABASE
... FROM FILE, because the database and log file backups must be local
to the SQL Server instance. Similarly, FILESTREAM is also not
supported at this time.
The BULK INSERT and OPENROWSET(BULK...) statements from the server are
not supported import procedures due to their dependency on the
ADMINISTER BULK OPERATIONS permission which is not granted for SQL
Server DB instances. Please use the process outlined below to import
data to a SQL Server DB instance.
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/SQLServer.Procedural.Importing.html