The main DSP of PSR 740 (102 effects) is only accessible when recording user styles. When making a style, at step to choose the R1 voice to the channel you intend to record, press Next button to choose the Group of voices and again Next to access to the DSP.

The insertion DSP effects (listed on pg. 156 of the manual) can be applied to any R1, R2, L1 voice (preset or XG) via the multi effects button to the right of the display. The preset voices usually have DSP effect assigned, where as the XG voices do not.

  The only DSP that can be applied to accompaniment voices is reverb & chorus (via Parameter edit). Fortunately, the variety of the preset voices containing DSP, all of which can be assigned to the accompaniment style voices, means that may useful effects really are available.

There are reported in the manual to be two types of DSP effects, system and insertion. Even though the manual appendix describes the system DSP (pg. 155) as being part of the style record mode, it seems to be a mystery how to modify the selection. Reading the midi sysex section carefully reveals that perhaps the system effects can be modified from here.