Are you getting the following warning in addition to the errors?
Warning The primary reference "LeapCSharp.NET3.5" could not be resolved because it has an indirect dependency on the .NET Framework assembly "System, Version=184.108.40.206, Culture=neutral, PublicKeyToken=b77a5c561934e089" which has a higher version "220.127.116.11" than the version "18.104.22.168" in the current target framework. UnityModules.CSharp
If so, then while annoying, it isn't fatal. Unity compiles the files just fine even though the IDE cannot. I only get the problem on Windows with Visual Studio, though -- not MonoDevelop (at least on the Mac). We inadvertently added a dependency to a later .NET version when we last built LeapCSharp.NET3.5. This should be corrected soon, but in the meantime, as long as Unity compiles the scripts, things should be fine.