The TCP sequence numbers are not exposed to any protocols above TCP by design.
Why do your dissector need to access the sequence numbers? Is it to detect retransmissions? If so the TCP itself already have code to do so. Is it to do PDU reassembly? If so the TCP layer again has code to provide this service to the protocols ontop of TCP. Is it to specify where in the sequence number space the next PDU starts? Again the TCP layer has a mechanism where a dissector may do so. Note that the sequence number itself can not be used alone to detect whether a packet is resent or not since a TCP will collaps multiple non-mss segments into a single segment during a retransmission. Thus a single retransmitted segment may contain partially retransmitted/lost data , partially retransmitted data that was not lost and new not-retransmitted data in the same single segment.
