One of our presidents here where I work at, personally assigned me to a project relating to mobile devices, I finished the project, then the president and I had a meeting where we discussed the success of the project, he then said it was great, and blah, blah, blah, and he told me that I was going to be transferred to work for one of his managers, I agreed (Of course I did), but the problem is that this manager just went to one of our customers to fix a problem they had in their system (he's going to be there for a whole week), and he didn't give me any task to do before he went to it. Now everybody in my team is assigned to something, except for me, I'm studying things related to the company area, but I feel useless, what should I do?
The first thing I would do would be to contact the manager, preferably by email (so you have evidence of the problem and your attempt to resolve it: CYA is rule one), and ask if there is anything he would like you to do while he is gone.
While you wait for a response, find something to do, even if it's just filing. There is always some task that needs to be done; ask co-workers or other managers for a short task if you cannot find anything yourself.
If you do not get a reply in a timely manner, or if the manager responds in the negative, contact the president who assigned you to the manager and explain that due to his absence, you do not have any assignments (either because the manager has nothing for you to do, or has failed to respond), and see what he would like you to do until the manager returns. Again, email is the best way to go here.