The interface protocol is specified in section 9.7 of the data sheet with a break-down of commands in section 10.
If you use it in two wire mode, join the DIN and DOUT pins together but be careful to change the pin direction on the PIC at the right time as it will be used bidirectionally and also note the need for a pull-up resistor as DOUT is an open drain connection.
Note that this is the interface to the HTRC110 to send commands or a single byte of data or send a command and read a single byte of data. There is an additional layer of protocol to communicate with a HITAG tag and it depends on the version of HITAG you are using.
Brian.