You don't have to place the formula on the report to be able to use it in the Select Expert.
Another way to do this would be to have the formula just return true or false. That way you don't have to compare it to anything in the select expert and the whole thing takes less processing. The formula would look like this:
isNull({Customer.Code}) or
{Customer.Code} = 'TEL' or
{Customer.Code} = '1MC'
This will return True if any of the conditions is met - you don't need "= true" or an if...then...else statement for it to work.
In the Select Expert you select the formula and "Is True".
Be aware that when you use a formula in the Select Expert, some or all of your report filtering will happen on the workstation instead of it occuring in the SQL that's run in the database (if you're using MS SQL Server, Oracle or another client/server database.) If you have a lot of data, this will take longer to run because all of the records will be pulled to the workstation and only the ones that meet this condition will actually appear on the report.
-Dell