Hello,
a dashlet that should be available in site dashboards needs to belong to the "family" "site-dashlet". You can specify the family of a dashlet in its *.get.desc.xml file. For the my-sites dashlet, simply copy the original my-sites.get.desc.xml to <tomcat>/shared/classes/alfresco/web-extension/site-webscripts/org/alfresco/components/dashlets/ and add a second <family> tag with the value "site-dashlet".
Regards
Axel