The Frame Rate control in the DCP tab sets the video frame rate that the DCP will use. Clicking Use best sets the rate to what DCP-o-matic thinks is the best for your content. With this button, DCP-o-matic assumes that the most commonly-working frame rates (24, 25 and 30fps) are allowed.
After this, the Video tab for each piece of content will give a summary of what DCP-o-matic is doing with that content.
If you want to experiment with other non-standard frame rates, you can do so by ticking the Allow any DCP frame rate in the Advanced tab of the preferences dialogue (see the Advanced preferences). You are strongly advised to use this only on your own equipment, and only for experimentation purposes.