Hi,
First make sure you have followed installation instructions for Aptana Studio 3:
http://wireframesketcher.com/install_details.html#aptana30
Afterwards, you can choose to create a Wireframe Project using File > New > Project... > Wireframing > Wireframe Project.
If you wish to add wireframes to an existing project then just switch to Wireframing perspective using Window > Open Perspective > Other ... > Wireframing and then create new screen from Project Explorer by right-clicking on a folder and choosing New > Screen.
Let me know if you need more assistance.