You must be doing something wrong. I just tested messui .135b and Color Computer(Extended BASIC 1.0) will indeed load tapes. Are you trying Color Computer or the Extended Basic when you say CoCo1?

The correct syntax, once the .cas is mounted is either
You don't need a name unless there are several files on the tape but you do need to know whether the game is in Basic or machine language.

Also RAM size is not an issue with a Coco1 but it should be 64K. You can use tapes with a Coco3 but you might need to "remove" the disk controller.