I was having some problems working this out and after several hours eventually solved it,
select the folder you want to share right click and select properties, then sharing tab and select advanced sharing,
tick share this folder then click permissions at bottom, all you need to have is administrators,
but make sure your log on account for windows is admin account.
Edit: It may help to add everyone permissions aswell but i found i did not need it.
click ok and go back to advanced sharing tab, take notice of the share name,
the folder i am sharing is My Pictures but the share name is only pictures.
now go to frontview and set up your backup job
remember what i said about taking note of the share name, as you can see above the path only has to be pictures
because thats how the share will show up on the network so your NAS will find it ok.
dont forget to put in your user name and password that you log onto windows with and all should work fine.
hope this helps someone it can be frustrating i know.