There is no convenient hitchhiking spot on the motorway in Unna itself. Unna borders Dortmund and it is only 20 minutes by train to go there. From there you have good hitching possibilities in all directions. Check the article of Dortmund for detailed information. Another option is to go to Hamm, what might be a good idea if heading to Berlin.
Have a look at the article of the Ruhr Area to see why it can be very difficult to find a lift exactly to the location you want to go inside this area. You should accept rides to cities next to Unna as Dortmund or Hamm and finally go to Unna by public transport.