The Attack Decay’s continuous envelopes tremolo effect does not have a tempo sync function but you might be on the right track with using the External Trigger input. Try disabling continuous envelopes and use the 8 step to trigger the envelopes. Set the Attack and Decay knobs so their timing matches the tempo from the 8 Step. If you change the tempo on the 8 Step, you will then need to change the timing on the Attack and Decay knobs.

If you have the Attack Decay set up to be triggered from an external clock, which would be connected to the EXP jack (see page 17 of the manual for setting this up), then each time it receives a clock pulse from the external device it will start the envelope sweep over. This is the only way we can think of to control the Attack Decay from an external clock.

You can set the Attack knob to zero or close to it so that the effect creates a short choppy decay that is triggered in time with the external clock.