If the first byte = 0x02, MESS will unscramble it.
If the first byte is anything else, don't know yet, needs to be checked out.
afaik, the RAW ones are encrypted, the TRANSLATED ones are decrypted, the ORIGINALS are what the guy received before he did anything. There are a number of bad dumps, don't be surprised if some things freeze or do odd things.
Now, I'm not sure what cassettes you may have, basically Tank or Invaders cassette is to be loaded through the monitor (with L command), press Enter and the new program appears in the menu. The other cassettes (pegapong, snake, startrek, chargen, charsave), you must start Basic, then use the LOAD command, then RUN.