i had the same issue, till i checked to see if my character was indeed cc3+

you'll need to ensure that it's using the facial profile that they mentioned in the link here
https://forum.reallusion.com/476420/Whats-New-in-Character-Creator-34#bm476426

I'm having the same issue with no blendshapes coming through, only the eye rotation and jaw movement. I noticed that the exPlus_remapu.asset file is empty. I've downloaded it 3 times to be sure, but there's nothing in there. I don't know if that's the problem or not.

You need to open up the actual ExPlus_remap and just resave / compile it and it will start showing up

It worked the first time I tried it, but the in another project I am facing the issue of Explus_remap going back to NONE in the Retarget Asset Dropdown --  just like other guys were.

The log says the following

EditorErrors: Error: Failed to load class /Game/LLF_Sample/ExPlus_remap.ExPlus_remap_C

Any clues on how to fix this path problem?

Thanks.

I have a same issue, using CC3+ still cant make it

I am experiencing the same issue. The model definitely has everything and I've gone through this 5x. Everything is good right up to getting the dropdown to appear. It doesn't.
I try and select ExPlus_remap and it reverts dropdown to "none".
The model was previously worked on with Substance Painter Pro - would that make a difference?
I'm stuck. Every step has been followed and happy to share my project. 

With the UE4.27 release there will be an "Auto Calibration" for the Live Link Face App.
Till it is released you can do it manually like in this video :

were you using CC3+ Base Character?

Cant choose ex_plus, i click on it but it changes back to "none"
all do like in video pls help!

Try this Tutorial to see if it helps!