First: Identify your contact
In a situation like this someone in the company was probably assigned to be in charge of you, or at the very least was pointed out to you as your contact within the company (a team leader, supervisor, manager, etc.)
This should be the first person you speak to, out of respect for the company's chain of command, and to show that you can follow simple instructions.
If someone was not pointed out to you, or if that person is not in the office (they're on vacation/sick/etc.) then approach the next manager up, or possibly Human Resources (HR).
There are now two ways to approach this situation, and they are not mutually exclusive:
Option 1: Officially communicate your concern
You have to be prepared to explain to the person you're speaking to exactly in what ways you could be useful to them, or the company/department.
If you've been assigned to a department (IT, for example), then explain to this person that you are eager to help, and list some of the ways in which you could pitch in. Maybe describe how you performed similar duties at your previous co-ops. Have a resume ready in case this person is not aware of your skills and qualifications.
Point out that since you're there they may as well use you.
If you performed software development, or database maintenance at your previous co-op but this company does not require those services then offer to perform low level IT Support (fix printers, help trouble-shoot network issues, etc.).
Don't be too proud to perform some of these low level jobs - even just being a receptionist, and forwarding calls. Be the coffee boy if that's what they need (and if you want to complete this co-op at all cost).
There is always some "dirty job" that no one wants to do. Even if it's scanning old manuals into a virtual library, or changing light-bulbs.
Note: If you speak to a team-leader/manager and they don't want to hear you out, or don't give you anything to do then you may wish to side-step them and go to HR, or a superior,
Option 2: Be proactive
Your actions here will greatly depend on the conversation you had with whomever is in charge.
If they hear you out and decide to give you a chance, then you're already set, but could use this approach to keep a little more busy.
If that person didn't help you however, this is going to be your last shot at being useful.
Basically approach people and ask if they need help. If you've been assigned to an IT department then maybe some people need a minor task performed. If the receptionist called in sick then maybe ask one of the managers if they would like you to "step in" for the day.
Note: This is tricky to do, because you could be stepping on people's toes politically. If someone gives you a task to do that a manager specifically asked them to perform, then you could be caught in the middle of an office conflict. Use common sense.