BIA uses JDBC to connect to the database(s), in order for this to work the appropriate JDBC driver needs to be available inside BIRT Designer Professional. The process for installing JDBC drivers in general is described in the "Using Actuate BIRT Designer Professional" manual, supplied with BIRT Designer Professional, for convenience the step required is listed below:
Copy the appropriate (Oracle or DB2 as required) driver jar files from $CURAM_DEVELOPMENT_HOME\CuramSDEJ\drivers to $ACTUATE_HOME\BRDPro\eclipse\plugins\org.eclipse.birt.report.data.oda.jdbc_XXX\drivers (where XXX is the version number, for BIRT Designer Professional 11 SP1 this is 2.6.2.v20110326-0603)