SQL Expression fields are only available if you are using a client/server database and then only through certain types of connections to the database (native works, but I'm not sure about ODBC, ADO, OLEDB, etc.)
To set one up, you have to use SQL that is valid in your database. I've used them as substitutions for formulas in certain circumstances when I want the processing to happen on the database instead of in Crystal.
For example, our database has a number of indicator fields that have YES, NO, or null values and null means NO. I've created SQL Expressions on these fields that translate the null to NO when pulling data from the database so that I don't have to do the translation in multiple formulas in Crystal.
-Dell