Where do I insert this code?

Dear Steffen, you will need to insert this code into php.ini

I get an error when I make that file.

Dear Steffen,  what kind of error are you getting?  ( can you paste it here or attach a screen shot).

Fatal error: Class 'PDO' not found in /home/datafbfg/public_html/ow_core/database.php on line 159

Dear Steffen ,

Fatal error: Class 'PDO' not found in /home/datafbfg/public_html/ow_core/database.php on line 159   .

This error means that PDO doesn't installed on your hosting.   It is a common issue. I highly recommend searching the forum if you get any issues in future. You might find the answer faster ;)

It is enabled. Talked to my host now.
Then why wont it work?

had the same problem some time ago....my host installed the ini file for me.
Ask your host to do it for you.....no stress  ;-)

I asked and they said I must to it myself.