Execute the following script using your MyGet [feedUrl] and MyGet [username] , [password] and [apikey]. Run this from a commandline where you have access to nuget.exe (or set the path to your nuget.exe in a system environment variable).
nuget setapikey [apikey] -source [feedUrl]
nuget sources add|update -Name [name] -source [feedUrl] -User [username] -pass [password]