The instructions that ship with the kit are a bit opaque for the Compute Module 3+ Dev Kit. I gleaned enough however from them to arrive at the following solution to boot a CM3+ Lite using an SD card I imaged using Balena Etcher (highly recommended for imaging SD cards for either standard Pi's or the Pi Compute Module's with eMMC onboard memory).
First, as there's (2) compute Modules in the kit, let's identify the CM 3+ Lite (the one with no eMMC on-board storage) so there's no confusion:
The kit ships with 4x pairs of jumper wires. We will use one of these to enable the Compute Module 3+ Lite to see the storage on the IO board that it's plugged into.
Next picture is with the Compute Module 3+ Lite inserted into the Compute Module IO board with an imaged SD card in the IO board's SD slot. Connecting the jumper wire to the 2nd pair of pins in GPIO Bank 0 enable the Compute Module 3 Lite to boot from the SD card on the IO board when micro USB power lead is connected to the "Power In" micro USB socket:
I achieved a correct result from the above procedure, but if there's any observed imperfections please advise and I'll update it.
If you need help imaging the Compute Module 3+ with eMMC on-board storage that ships with your kit, please follow these instructions which are quick, painless and proven to work:
How do I Flash (install) Raspbian on Raspberry Pi Compute Module 3 / 3+?
Hope this gets folks up and running with their new Compute Module 3+ Dev Kits which just started shipping yesterday-