Welcome to the ODK forum, @elda! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!
Are you able to attach your survey form file here (with what you've tried so far)? That will help us troubleshoot. If you are using ODK Build, you should be able to save it as an XLSForm file.