What file would I need to modify.
This is not a quick answer really. You would need to create a DataSourceItem that inherits from the DbDataSourceItem class. The best thing would be to copy over the SqlDataSourceItem and start from there. That would be the closest data source to MySQL. Then you will just have to start tweaking it until it is ready for MySQL.
Will the StrataFrame Business Mapper work the same way as for the MS SQL.
No. You would have to connect using the OLEDB provider for the time being, which by the way, is also a way to talk to MySQL right now without creating a custom data source item.
Adding the native implementation into the BO Mapper like the DDT, SQL Server, Oracle, and DB2 will require a change by us to make this work.
Like I said before, I think that it is likely that we will create a MySql data source item, but I cannot give you an exact date of when this will happen.