Once you have created Rivka as the child of Sura and Meir B as the child of Dov select Rivka and click "Add partner/ex". Then select "Partner with person already on tree". You will be presented with a drop-down list of people on the tree, this is where you select Meir B. You must then click on either "Add as current partner" or "Add as ex-partner" to make the connection.
Hope this helps.