A milestone is a task of zero "work" that shows an important achievement in a project. Milestones are a way of knowing how the project is advancing if you are not familiar with the tasks being executed. They have zero "work" because they symbolize an achievement, a point of time in a project. You can actually report time and actual effort on the milestone but since it symbolizes an event, you cannot define "work" for a milestone. Therefore, the work field will be locked for the milestone. Let me know if this helps.