I don’t know the JERS data, but I guess the tif files do not provide enough meta information to be used with the SAR-mosaic.
This does not work because the area you of your data is not located in the default area of the mosaic.
Try the Mosaic in Raster/Geometric Operations from the menu.
The XML you mention is not shown.
At the end, it can turn out that this JERS data you have is not supported in a way you need it. As far as I know, JERS is only supported in CEOS format.
Update: I moved this to a separate thread