You can use the Char function to return the character for a specified ASCII. Code. You can look up the code function in Filemaker help for a short list of commonly used character codes.
¶ can be used for returns in calculations. (There's a button for inserting it in the Specify calculation dialog.)
If you want to enclose " inside a quoted string, you can precede it with the backslash: "This is a \"string\"" will evaluate as This is a "string".
The Quote function can also be useful in these expressions: "This is a " & Quote ( "string" )