To add a plugin to SAP2000, select use the "Tools > Add/Show Plugins..." menu command.
If adding COM plugin, Plug Ins, then enter the exact name of the plugin, which would typically correspond corresponds to the name of the plugin DLL file. Finally, enter the text which will appear on the Tools menu.
If adding .NET plugin, click the "Browse" button to browse for and select the plugin DLL file. Then click the "Add" button. Please note that .NET plugin support is available starting with SAP2000 v22.2.0. For earlier versions of SAP2000, a .NET plugin can be registered for COM and used as a COM plugin.