The zip contains and SNA a BMP and a TXT file. The Sna is dual compatible between original 48k and Next. To use on a next, create a sub folder and place all files inside it and run the Sna file from the next browser. Instructions are in the TXT file on how to do this.
maybe things have changed in cspect since i wrote it? The sna self detects if it thinks its running on a spectrum next so there is nothing to do. Ensure when you run the sna under the next browser or cspect command line ensure to choose next rom and not 48k rom to run under. Ensure that the bmp file is also in the same directory. When you run and quit the game on the next it should also create a hi score save file in the same folder after playing the game. I dont have my systems setup anymore to test so just going off memory here, but other people have run it succesfully from other screen captures on youtube i have seen.
Cursory glance at the cspect readme i guess you will need to run cspect with command options -zxnext -nextrom -mmc=c:/path to folder/ ???