Select all desired lines from a source file (or all lines with Control-A) and press Control-C (copy). 
Then go to the input textbox on the DT5GL tab and paste the copied text with Control-V.

If source text is already present in this textbox, select all text first with Control-A, and then overwrite it with Control-V. 

In short: 
in the source text file: Control-A, Control-C
in the input text box: Control-A, Control-V

The text analyzer (parser) of DT5GL only responds - case insensitive - to the keywords 
- Table, rTable, 
- Proposition: , GoalProposition:
- Attribute: , GoalAttribute:
- Database_view:
and the desired subsequent keywords, so all other text will be considered as comments. 

Thus all text for the first 'Table' or 'rTable' in a source file is always comment and 
can easily be copied to the input field on the DT5GL tab (without any error messages).

When creating your own source files, Notepad++ is highly recommended. 
It can be downloaded free of charge from: https://notepad-plus-plus.org/

Use a monospaced font for correct alignment of the decision tables, 
for example: lucida console or courier new. 

If the first line in the source text file is also the first line in the input text box of DT5GL 
the line numbers in any error messages will correspond exactly with the line numbers in Notepad.