it is in the valve body, accessed by removing the pan, something best left for a transmission shop, not because difficult, but very very messy and time consuming without a hoist etc. Maybe do all the solenoids while you are there, or even a shift kit! My solenoids were replaced as part of my shift kit mod..
bottom of first page/ same tranny as S1 Stageas RE4R01A
http://nissan.damagedreality.com/NissanTransTests.pdf