That service does not provide access to the file system. If you wish to add more plugins to your forum you either have to:
a) ask the administrator of the service.
b) install MyBB on a web host which does provide file system access.
If i install mybb can i still use my forum?
If the administrator of the service will provide you with a backup of the database then yes, you can continue to use your current forum. Else you'd have to start from scratch.
What does that mean? how do i contact them? if you mean admin of the forum , then thats me
admin of the service you are registered on: FreeMyBB
Ask them if they can give you a backup of your forum.
With a backup you can "copy" the forum in your own hosting. And thus install plugins cause you will have FTP access to your files.
Contact the FreeMyBB service and ask them if you can get the plugin installed. if not, ask them for a database backup if possible.