daisylove,
if you do mysql -u mydatabaseusername -p (enter) at a command line, do you get a password prompt, and once you've put the password in can you do "use (roundcubedatabasename);" (enter), "show tables;" and get results? Replace (roundcubedatabasename) with the name of your roundcube database, and I'm assuming that the host part is set to the hostname of the system you're accessing the database from ie. where your Roundcube install is, and that yuo can test this on same system's command line.
If it logs you in but wont let you do anything wiht the data, then "grant all on (roundcubedatabasename).* to 'mydatabaseusername'@'hostname' identified by 'password', making the substitutions for hostname, password etc.
what o/s are you on?
Andy