Maybe a little late to the party but since majority of the answers are very old; just wanted to share the command that works right now :-
So, to manually mount the s3 bucket using an IAM role you will need to fire the following command :-
$ sudo s3fs <bucket-name> /<folder-path>/<folder-name> -o iam_role="<Role-name>" -o url="https://s3-<region-name>.amazonaws.com" -o endpoint=<region-name> -o dbglevel=info -o curldbg
Now, for example if my bucket-name is "Test-Bucket" and the folder path is "/root/Test-Bucket" and my IAM Role name is "ec2_to_s3" and the region in which I have the s3 bucket is "Asia-Pacific Mumbai", then the above specified command will be used as :-
$ sudo s3fs Test-Bucket /root/Test-Bucket -o iam_role="ec2_to_s3" -o url="https://s3-ap-south-1.amazonaws.com" -o endpoint=ap-south-1 -o dbglevel=info -o curldbg
and the bucket will be mounted successfully.
Now, if you want the bucket to get mounted on system reboot by its own; you will need to add this string in "/etc/fstab".
<bucket-name> /<folder-path>/<folder-name> fuse.s3fs _netdev,allow_other,use_path_request_style,iam_role=auto 0 0
Now, as per the previous example, the string that needs to be added in "/etc/fstab" would be :-
Test-Bucket /root/Test-Bucket fuse.s3fs _netdev,allow_other,use_path_request_style,iam_role=auto 0 0
Hope this helps...