Are you trying to export using the button in the Surround Encoder window or from the File menu? If from the File menu, it won't work, you need to use the Export button in the bottom right corner of the Surround Encoder window. This displays a dialog box called Multichannel Export Options, which allows you to save as 6 individual waves, one 6-channel interleaved wave file or one 6-channel Windows Media Audio Pro (compressed or lossless). This Encoder window is accesible when in Multitrack mode.