@nina : To add an excerpt to a post, simply write one in the Excerpt field under the post edit box. Some screen options on the Post & Page edit Administration Panels are hidden by default. The Excerpt field is hidden by default if it has not been used before.
Screen Options, displayed as a hanging tab under the toolbar, allow the user to decide what fields or modules are presented in the work area for a given Administration Screen. Each Screen may have a different set of Screen Options.
Click on the Screen Options tab to expand the options available for a particular Screen, check on Excerpt under boxes, then click the Screen Options hanging tab to collapse the Screen Options.
Let me know if you have any further issues.
Regards,
Tikaram