Anyway, XBMC seems to have a problem with SMB shares on Synology when DLNA is activated on the NAS. You can disable DLNA and try again. If you need DLNA activated you can try this link to set up NFS on XBMC. It worked for me.
It that can helps, I have an ATV with ATV Flash 1.5 and all my movies are on my Synology. DNLA is activated (Synology) and I access my “video” folder from my ATV, it works just perfectly, I don’t understand why Apple didn’t activate this DLNA feature in the original Apple TV (in fact to force people use Itunes…)