Heterogeneous queries are supported in the Professional edition of Toad Data Point, so make sure you're using that edition (Help | Licensing from the main menu).
Secondly, is the Oracle table not displayed in the query builder? With the Query Builder as the focused doc tab, try double-clicking on the Oracle table within the Object Explorer. This is an alternative way to populate the query builder with objects/tables.
If the Oracle table does not display, either by dragging/dropping it onto the Query Builder, or double-clicking the object within the Object Explorer, then there's likely something wrong which would warrant a call to our Support Group.
Regards.