My own inclination would be to visualize these using MRIcroGL (see attached image where the output of probtrackx is loaded as an overlay and the glass-brain overlay is used). In my own work, I tend to turn the output of bedpostx into streamlines that can be visualized as Tracks in Surfice (similar to running the "Track" demo from the Scripting/Pascal menu item). 

If you really do want to visualize these with Surfice, you will need to choose a single surface intensity for your probtrackx output with the Advanced/ConvertVoxelwiseVolumeToMesh option and view these. You would then use the "XRay" feature to make the cortex translucent so you can see the fiber bundle beneath:

BEGIN
RESETDEFAULTS;
COLORBARVISIBLE(false);
MESHLOAD('cortex_mesh.obj');
OVERLAYLOAD('fiber_mesh.obj');
SHADERXRAY(1.0, 0.2);
END.