can’t really tell much without knowing this package, but ./ is not what you’re looking for. try just “make” or “make clean”, as this is standard syntax for Makefiles. if you are wondering what happens, make looks for a file called “Makefile” in the current directory and executes whatever is inside. in your case it will most likely compile the .c file into an executable
In that case I recomme d the Connections Museum Youtube Channel. They have tons of working vintage telephoby equipment, including a DMS-10. This might even be one of their photos.