Hi,
the .tsc is a TINA schematic file that you can open with TINA or TINA-TI. If you want open a schematic file of another platform with Multisim, you have to import the netlist of the circuit. In the nimultisim videotutorials page you find the video “Import a SPICE list” where the two methods to import a netlist in Multisim are well explained: the SPICE block method where you can paste in the text of the netlist to be simulated and the Netlist importer method.