Right click on InfoSource and you can see Assign DataSource or
Right click on DataSource and you can see Assign InfoSource.
In BI7 you can find Create Transfer Rules click on that
Just right click on your Info source and you will find assign data source. Click on it and simply write the data source name.
You can also do it from the data source. Follow the same proc, right click and this time click on assign info source,
Hope this helps.