While the command works fine under GnuPG on Linux, it fails in Windows. I've been able to make it work by removing all spaces:
gpg --keyserver hkp://keyserver.ubuntu.com --recv-keys 843938DF228D22F7B3742BC0D94AA3F0EFE21092 C5986B4F1257FFA86632CBA746181433FBB75451
(the quotes are not required any more, either; so I removed them).
If you really want to verify against signatures from Ubuntu, you obviously shouldn't copy/paste this specific command line as is, but remove the space from the original Ubuntu page on your own.