If you are using the lastest beta release of build 1278, you need to apply a patch. Please follow these steps:
1. unzip the attach zip file.
2. copy the unzipped xmailManage.tag file to override the copy at <install>/jetty/webapps/zimbra/WEB-INF/tags/desktop/xmailManage.tag, where <install> is the folder you chose to install zdesktop.
3. delete everything under <install>/jetty/work but not the "work" folder itself. On windows you'll need to first shutdown the "Yahoo! Zimbra Desktop" service before you can delete the files there.
This patch is especially important to Yahoo! Mail users. Without it, you can not change settings on the Yahoo! Mail account setup page.