There are a couple ways to do this but I think the easiest to follow is this
....
You don't need to worry about the task type or anything like that, the
default "fixed units" type will work fine. Remember duration is the time
span from when work first begins until when it's done - it is NOT the
"window" during which the work might be done before some deadline. So
saying your task is 2 weeks duration and requires 16 man-hours of work means
that the resource is going to be spending a part of each day on it, but not
his full work day, spread out for a period of 2 weeks. For him to
accomplish 16 hours of work (2 workday's worth) output spread out over 2
weeks (10 workdays) means he is devoting, on average, 20% of his energy each
day to the task. Thus over the course of 10 days he creates the same output
that would have taken him 2 days if he had worked on it full-tilt. So enter
the task with either a 10 day or 2 week duration, they're the same thing if
you work a 5 day week, and assign the resource at a 20% assignment level in
the resource assignment window. Project will calculate the task's work for
you to be 16 hours and you're done.