Eliza,
You need a separate number pad to use the ALT codes, like the number pads in cash machines, at the right-hand side of the keyboard on bigger laptops, so next time you buy one ...

If you use a Word document, click on 'insert' at the top, and then on 'symbol', and you will have lots to choose from. (Then copy and paste into the mudcat message box.)

Or you could try these in a Word document, but they don't seem to work in the mudcat box:
Hold down CTRL and press , then release CTRL and type c for ç
Hold down CTRL and SHIFT for : then release CTRL and type e for ë
If you have an ` you can hold down CTRL and press ` then release CTRL for à
Hold down CTRL and SHIFT for ^ (over the 6) then release CTRL and type o for ô