How to redirect to a node/add page after clicking on a flag link
Ok, I myself test this for you, please first install and enable Rule and Rules UI  ,after it (I suppose you have a flag with 'myflag') .
Go to dmin/config/workflow/rules  and Add new rule ,set your name and in  React on event  in flag group select A node has been flagged, under "myflag"  save it ,after it in Actions section  choose Add action , in Select the action to add in system group choose Page redirect ,after it set your path want to redirect it in Value. (is better check Force redirect). 
I hope works for you correctly.