It's designed to limit the color ramps shown depending on category. Category is an attribute you can edit so you can change it for color ramps you want to show up. To edit the category of a color ramp do this:
1. Open the Styyle Manager (Customize menu > Style Manager)
2. In the Style Manager find the Color ramp folder in your style in the left hand pane and click on it to open it
3. In the right hand pane find the color ramp you want to show. Make sure you have the style view set to show details.
4. In the Category column click on the category text next to your color ramp to put the text into edit mode.
5. Change the text to Default Ramps
OK out of the dialogs and you should see your ramp in the layer symbology dialog dropdown now