Simple shaft encoders use two identical pulse trains, say A and B, from the
encoder that are 90 degrees apart in time. That way the direction of turning
can be detected depending on whether the A pulse is output before the B
pulse. If your circuit has two data lines going into the PIC (not counting
ground and logic power supply lines) then it is probably one of these.
Grounding the A input then the B input should make the frequency go one way
(say up) and doing the opposite, grounding the B input and then the A input
should make the frequency go back (go down).
If this works then you can make a simple logic circuit to do it but beware
of trying to do it too fast. The DDS may not be able to respond at 45 bauds?
Hope this helps.