On Linux: You need a kernel with IPv6 support (all newer kernels 2.4.x). However, Ipv6 support is usually compiled as a module. Try calling 'modprobe ipv6' command to load IPv6 support.
On Windows: Microsoft have official support for the implementation of IPv6 on WinXP SP1 and newer operating systems.
Systems prior to this version sometimes have 'preliminary support'. This version is for developer testing. It is not release quality. Not all needed functions are supported in these versions!
Synapse supports these preliminary versions too, however autodetecting of socket type does not work properly in all cases. A better way is tp set the socket Family in your source to SF_IP4 or SF_IP6.