That is because you do not have access to the newly created database. A quick Google search can give you some answers: http://dev.mysql.com/doc/refman/5.1/en/access-denied.html
Changing the DB prefix and importing to a database that is already there is going to be tricky because it is not really designed to do that. It is designed to restore form Dropbox to a blank host.
To do this you will need to get your tech hat on. Once you have the access error sorted your will need to alter the SQL dump to update all the inserts to replace your old table prefixes with your new ones.
If your blog is brand new and you dont care about the data and the backup is the same WP version of what is installed you should be able to truncate the tables and then import the SQL dump.
As mentioned in my other post, restoring is a difficult problem, especially when moving to new hosts! It's something that I am planning to solve for users who cannot use Simple Restore in the coming months.