There are occasions when you want to capture packets using tcpdump rather than wireshark, especially when you want to do a remote capture and do not want the network load associated with running Wireshark remotely (not to mention all the X traffic polluting your capture).
However, the default tcpdump parameters result in a capture file where each packet is truncated, because most versions of tcpdump, will, by default, only capture the first 68 or 96 bytes of each packet.
To ensure that you capture complete packets, use the following command:
tcpdump -i <interface> -s 65535 -w <some-file>
You will have to specify the correct interface and the name of a file to save into. In addition, you will have to terminate the capture with ^C when you believe you have captured enough packets.
tcpdump is not part of the Wireshark distribution. You can get it from: http://www.tcpdump.org for various platforms.