I have an ArcPy script tool that exports a map with appropriate labels (10.1 SP1) and my coworker who made the original mxd that I'm basing it on is using Myriad Pro as the font for the text elements. How might I get this font in my mxd? Or could I simply get her to create the mxd which the tool will reference?
Fonts are part of the Windows OS, not specifically ArcMap.
In order to add a font to your system, typically you would have a .ttf (TrueType Font, seems to be the most common) or.otf file. To install it, you simply double-click the file and follow the prompts. I think Myriad Pro is bundled with Adobe Reader and the files are located in the Reader install dir, under Resource/Font. You could install them from there. However, as far as I know, this is a commercial font. You should consult any EULA or other licensing documentation you can find that came with Reader to see if you are allowed to use the font the way you intend to.
The font does need to be installed on the computer you are opening the MXD on, otherwise it will probably just be replaced by one that is installed in that computer, even if your coworker sets up the MXD with a different font.
Refer to this help page. You are going to need the true type font file for Myriad Pro, a .ttf file. Following those steps will add the font to ArcMap and allow you to use it in your .mxd