Well, in Interface maker, the only main thing it allows you to do, is to create custom layouts. That being health, ammo, etc ,etc can be moved around.
If you wanted to make a new custom interface, you would have to-
1- click the 'new' button. this would create a new custom interface folder named '(your title)' in the custom-interface folder in soldat.
2- Manually edit each of the HUD elements of that particular interface. Note tho, the interface maker ONLY supports 640X480 resolution.
3-Once done with that, you could rearrange in an order you'd like and even adjust the Alpha settings.
4- save and your done.