The old FSharp Powerpack seems to be obsolete, and most of its components have moved to other projects. Where can I find the ArgParser module? Is it still maintained?
An improved version of ArgParser is maintained as part of ExtCore. If you're looking for a library of extended library functions, ExtCore is an excellent choice.
If you don't mind a few more files, just put Arg.fs and Arg.fsi into your projects. They're self-contained; I did it here without issue.
As the comment above suggested, you should give UnionArgParser a try (see their docs at http://nessos.github.io/UnionArgParser/). It is a well-maintained and fully declarative alternative; that is much better than ArgParser.