I have a DAG card but libtrace is not detecting it during the configuration stage?

You can manually specify where your DAG libraries are located by using the --with-dag=DIR option for ./configure. DIR needs to be the location you installed the DAG software to, not the location of the DAG device e.g. DIR might be /usr/local/dag not /dev/dag0.

tcpdump can't read the pcap traces I created with libtrace!

By default, pcapfile: outputs compressed pcap traces which tcpdump can't natively read. Use tcpdump -r <(zcat mytrace.pcap.gz) to read these traces.

libtrace doesn't understand header type x!

If you are trying to use libtrace to understand a header which libtrace doesn't natively support then you can use getlink to get a pointer to the link layer (or some suitable other header), and then use trace_get_payload_from_header to skip past any extra headers you might need to (for example vlan headers), then take the result and cast it to a structure representing your preferred header.