Hello,
1.You can activate an inactive project by directly changing its start time. As long as you change its start time into Today or Next, it will be an active project.
2.In the project list on the right,right click on the project and select "Activate" button to activate the project.

You could also click the following link to know more about it:
http://faq.doit.im/web.html#85

Thank you for using Doit.