Solution
One-time setup; go to:
[4. Miscellaneous] - [4.3. Settings] - [4.3.3. Company]
Check the box for the 'Free text' option.
To enter free text for matings, go to:
[1. Enter/Edit] - [1.1. Sows] - [1.1.5. Mating/farrowing/weaning data (individual)]
On the 'F5 - Matings' tab, the fields where you can enter free text are visible under 'F5 - Mating comments'.
Note! The content of these fields is not printed on the sow card! Free text can only be entered via option 1.1.5.