Ideally, an HVAC professional. I'm far from an expert, however, the task is often easy enough for anyone who's the least bit handy. A 5' length of 4" metal ducting costs $7 at Home Depot. You may need a couple of 90-degree turns, and an outside covered vent--all told, under $50 in materials (the most expensive part is the metal ducting tape that costs about $15/roll). You may also need a 4-1/8" hole-saw and a drill (or some other cutting tool) to cut a 4" hole into your siding.

If you need the cosmetics of an in-wall vent installation, that's a bit more challenging (needing an even "handier" guy/gal), and requires more specialized ducting, which is slightly more expensive, and not carried by Home Depot.

This post was edited by studio460 on Sun, Jun 2, 13 at 6:00

Are you the landlord or the manager? If you have not already done so, you need to perform remediation of the mold affected area. Inspection could very yield the need to further pull drywall away from the studs and to clean and replace materials in the affected area. This is also a good time to review where a dyer vent can be established. You will need to contact your city's Building Department regarding building code as they apply to dryer vents and gas /electrical requirements for laundry spaces.