Is your parameter a string or a number datatype? If it is a string, then add an option "All" to the picklist, choose "allow multiple values" and then go to report->selection formula->record and enter:
(
(
{?Parm} <> "All" and
{table.field} = {?Parm}
) or
{?Parm} = "All"
)
If it is a number, then add a value option 0 with a description field of "All" and then also add the other picklist options as description fields. Then choose "true" for "show description field only", and allow multiple values. Then set up your formula like this:
(
(
{?Parm} <> 0 and
{table.field} = {?Parm}
) or
{?Parm} = 0
)
-LB