Dear Marcel,
If you are using Types URL field (http://wp-types.com/documentation/functions/#URL) that allows inputting/outputting of URL, then you can try the following:
<a rel="nofollow" href="[types field="website" raw="true"][/types]">This is your link text</a>
The technique is to set raw equal to TRUE so it will output plain URL (no styling, no HTML, just the URL as stored in dB). Then use that to form into a hyperlink. In the hyperlink HTML, you can then easily put nofollow attribute. Please let me know how it goes.
Cheers,
Emerson