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.
if you are completely unfamiliar with C, but are using matlab as your base starting point, i would actually recommend fortran90 over C. the languages are so incredibly similar it is scary (as far as syntax goes at least)
You may want to look at this series of articles on scientific computing with Cocoa from MacResearch.