If the fixed cost is each time you use the resource, it costs say, $200, then on the resource sheet there is a Cost/Use column.
If the fixed cost is related to the task, but not associated with a particular resource - then, while displaying the Gantt Chart, view the Cost table. There is a Fixed Cost column in that table.
Lone84 - I don't quite understand your advice. Setting units to 0% on the Resource Information will make that resource unavailable during the specified time period on the Resource Information page. This will cause the resource to show up as overallocated.