Just adding the border radius lines to your CSS shouldn't affect your saved new preset in any way. All that should happen if your new new preset were used again, is that it would overwrite any changes you made to the stylesheet outside of the CSS Sculptor GUI, i.e, your rounded corners.
Check that your page is linking to your correct stylesheet, by checking the code view of your page.
<style type="text/css" media="all">
Are you using CSS Sculptor or Site Sculptor? I have tried both, added the rounded corners afterwards to the css, and in each case my preset remained intact. If you are using CSS Sculptor, check that you are getting back to the correct preset by checking the name of the folder inside the CSSSculptor folder, - it will be named (something like):
which shows the path to your own saved preset (where 'MyNewPreset' would be the name you gave your preset).
The easiest way to achieve what you are after, is to use CSS Sculptor to create the basic page elements as before - using the preset closest to what you want to achieve, and then add the finishing touches - such as rounded corners - afterwards.
If you use (say) - 1ColumnFixedCenteredHeaderandFooter - you would add the navigation bar to the header area.
If you do create a page and are able to upload it to your remote server where we could view it, we could more easily advise how to achieve the styles you want.
There is also a tutorial for adding image-based rounded corners - along with downloadable images and presets - which you should be able to access from your downloads page / CSS Sculptor / under 'support resources' and 'source files'.