Step 1 is to open up the pcap file in Wireshark and take a look. Searching for "pass", there are several instances, but one in partiuclar is of interest. There is an upload of a file called pass.zip. This is the file I want to extract. There are lots of ways to extract files from tcp data (e.g. tcpxtract), but I chose to do it manually.
In wireshark, I highlighted the "Media Type: application . . ." line to select the file data, pressed <ctrl>h and saved the file to c13.zip.
Now that I've got the zip file, I have to figure out the password to it. I generated a dictionary based on the challenge parameters and used fcrackzip to find the password. Unzip the file, cat out pass and the challenge is done. It's important to use the -u switch with fcrackzip as without it, there are about 50 potential passwords listed.