I'm having a tough time understanding what you mean exactly by open on top. It sounds like you're describing what Edgecam's feature finder would find as an "open pocket". Did you have "open pocket" selected when you ran feature finder? You may want to select every type of feature, then run feature finder to see what you get.
If you still can't find a feature that will correctly create toolpath, you may want to select the entire model, and use a roughing cycle. Set your Z depth to the deepest Z dimension in the pocket, and create a containment boundry to confine the toolpath to the pocket.
You can easily create a containment boundry by using the "create geometry from edges" to select the outermost edges of the pocket. On the open side of the pocket, you can translate or offset the your boundary away from the part to allow the tool an area to approach from. The containment must be closed.
I hope that helps.
I'm just guessing at what the problem is. Some screenshots would really help, and what version of Edgecam you're using and your level of experience.