string.Format() monodevelop error

Hello

this is driving me nuts (sorry).

I wanted to format a double using string.Format(). Mono develop reports it as an error and cannot autocomplete. If I save the script unity3d compiles it ok but mono develop still freaks out with it.

Also Mono develop won't allow a using System.String - I understand string is aliased but is that right that I cannot do the using directive??

I am using the latest free version of unity3d -4.3.3f1

Thanks in advance

Robert