In order to put text on your card, you'll need to add a Field widget.
Switch to widget mode with Tool->Widgets
Then create a field with Widgets->New Field
The field widget will appear and you can position and resize it as needed
To type text in it, double-click it and type into the Text section of the properties box. Alternatively, if you switch back to Interact mode (Tool->Interact) you can just click in it and type.
(As a final tip, if you're looking at releasing a deck for others to use, you may want to "lock" the field to stop the text from being editable in interact mode. You can do this by selecting Widgets->Locked from widget mode with the field selected)
I hope this makes sense, let me know if anything doesn't though!