Go to the field explorer->parameter->new and create a parameter
{?custno}. If the customer number is a number, choose "number" as the datatype, otherwise select "string". If you want the ability to select multiple customers, then set that option to allow multiple values (or in higher versions, set multiple values to true). Then go to report->selection formula->record and enter:
{table.customernumber} = {?custno}
-LB