Just doble checked and you cant add Polar Flow or the web service on MMR (or MFP). There is the option of a sync with polarpersonaltrainer but that is an old web service and doesnt support the V800 as they are wanting everyone to move to Flow.
Anyway, I have found a work around, there is an app called syncmytracks on android which (and this is a weird work round) that I can sync my polar flow with garmin connect which will then sync with MMR and MFP. just for the peopel that may be searching this thread.
I got a nice shiny new v800 for my birthday on Friday and took it out for a spin yesterday. What I can't work out is how to sync polar flow or the Web service data with myfitnesspal. I was able to do tcx file transfer to mapmyrun but it doesn't seem to have synced that way either. Ideally looking for an android app (flow) to mfp or done through the Web service? Having read up a bit on this it seems that the android sync isn't available yet, only ios but really