The easiest way to install web parts is probably to use this utility from Microsoft called Install Assemblies - very simple and intuitive interface, takes care of creating the Safe Control entries and even creating DWPs for your web part assemblies.
Install Assemblies internally uses STSADM.EXE - more than often you will face the infamous "The server instance specified was not found. Please specify the server's address and port." error. There can be many reasons why you will face this problem, here are top three reasons I've seen in the field either while installing web parts or while using Object Model code -
1) WSS SP1 not installed on all components
Diagnosis: Go to SharePoint Central Administration -> Click Windows SharePoint Services -> Click Extend or Upgrade a Virtual Server (Second Link) -> In the virtual servers listed, do you see "Upgrade" next to your SharePoint virtual server.
Resolution: Upgrade all your components to SP1 using the STSADM.EXE util -> follow steps listed in this KB article
2) You receive the above error when using SharePoint Object Model code and using the Hostname/Fully Qualified Domain Name (FQDN).
Resolution: Follow this support article http://support.microsoft.com/?kbid=832816
Resolution: Use the IP address with https:// instead of the sitename (eg SPSite testSite = new SPSite("https://220.127.116.11");
- Mohammed Jeelani