It will only take a few seconds to install and you will get a message when it is finished.In the black window write: regsvr32 steam_api64.dll and then press Enter.
If you don't know, what is DLL - read wikipedia article about Microsoft Dynamic-link library How to install steam_api64.dll file? Dll usually should be in the folder C:\WINDOWS\system32, C:\Windows\SysWOW64\ or in the application folder in which the error occurs. If you have an error related to steam_api64.dll, check there is a file on the computer.
