I think the mosaic operation was available in a previous beta version. Maybe it was deactivated for some reason.
However you can write the graph file manually and use it for batch processing. You can also use the mosaic from Raster/Geometric Operation/ Mosaicing to define the parameters.
There are two mosaicking operators developed by two teams.
The Raster/Geometric Operation/Mosaicing currently does not have a UI compatible with the graph builder. As Marpet has said, you may still define your parameters in xml and use the command-line gpt.
The other mosaic oeprator is the Radar/Geometry/SAR-Mosaic which works similarly but has been developed to handle SAR bands and polarizations.