I too had considerable difficulties with getting the Sentinel Toolbox but it finally came through with some help of Canadian colleagues (it was developed by a Canadian outfit, drawing upon years of Radarsat experience). I think we may have been stymied at the same point. The Terrain Correction module accesses DEMs from an ESA FTP site which was not possible on the network. You can test for this by going to Help > Test Connectivity and seeing if the FTP connections go through. I had to create a workaround to do this by downloading all of the DEMs separately from the site and mosaicking together the appropriate DEMs for the Radar images.
In any case, it is true that the error messages are not very helpful.