There's two ways to run tcpdump depending on whether you want to write to a file or not.
Firstly you can access it from under the "diagnostics" option in the CLI.
diagnostics -> network -> tcpdump
The output files will be written into the "diagnostics" directory that can be accessed via FTP or SCP.
You can also access it by running "tcpdump" directly from the CLI, but this doesn't give the ability to write the output a file.