If Adobe Experience Package Manager does not load (for example, if you encounter a 404 error while attempting to open it), the first step is to check the log. Ensure that the required JCR nodes are correct. If these nodes are not present, you see an error in the log file. For example:
log: 27.06.2014 13:16:53.845 *ERROR* [127.0.0.1  GET /crx/packmgr/list.jsp?_dc=1402543013769&_charset_=utf-8&includeVersions=true HTTP/1.1] com.day.crx.packmgr.impl.servlets.ListServlet Error while retrieving infos: javax.jcr.RepositoryException: Invalid path:/etc/packages/my_packages/.snapshot/My Packagename
See this community article for more information: AEM Gotchya: No packages in Package Manager.
Check what URL you see when you hover over the packages link on Welcome screen. Does it show http://localhost:4502/crx/packmgr/ or http://localhost:4502/crx/packmgr.html? If it shows .html on the link, you could have a rule configured under etc/map that could be causing this behavior.
Finally, check the permissions in "/libs/cq/core/content/welcome/features/packages." Ensure that it has all the required permission levels for the Adobe Experience user to access Package Manager. A non-admin account needs the required permissions to install a package.