Importing System.Resources, but still getting error "Type 'ResxResourceWriter' is not defined"

We're migrating a classic ASP application to an ASP.NET webapplication. In the old application we had our language strings in an ASCII file and to save time I thought I would try to generate a RESX file dynamically from that.

First I tried using the ResourceWriter class, but then I don't get the XML type resource file that I'm looking for.

Now I've Googled around somewhat more and I understand I need to use the ResxResourceWriter class, but the compiler can't find it.
It should be located in the System.Resources namespace, which I'm importing, but I keep getting the error "Type 'ResxResourceWriter' is not defined".

Any ideas? Thanks in advance.