Add a reference to your project: open the solution in the Solution Explorer pane, open teh project branch, and right click the References twig. Select "Add a reference" and then browse to the DLL file.
Provided it is a .NET assembly, it will be added and you can use it.
If it isn't, then you will get a error message, and you can't use it directly.
If that's the case, try this version: (AGauge) WinForms Gauge Control
] or this: An Improved Version of AGauge (A fast and performing gauge)
] in case your DLL file is corrupt.