I want to get started on scientific programming on the Mac using Objective-C. I am very familiar with MATLAB which makes it easy to store complex-valued waveform data in vectors and generating plots. I was wondering if there might be a good resource that might compare how you would implement common MATLAB operations (i.e. creating an array of complex values or generating a mesh plot) in an Objective-C/Cocoa environment.
I don't know of any Objective-C-specific libraries or frameworks, but there are a large number of libraries written in and available for plain C, and you should be able to use them with Objective-C.
Using C allows you to use PETSc, gsl, LAPACK, and loads of other useful numerical libraries. I would advise against trying to code the matlab routines yourself. Internally, matlab likely uses well-known numerical libraries itself.
You may want to look at this series of articles on scientific computing with Cocoa from MacResearch.