I have many problem with my ngx:

1. The FD don't disappear at 50 ft during ILS approch

2. During fmc programming at the gate, if I preselected SID, I can't change it.

3. I have problem to create along track waypoint like this video: 

If I have runway without R,C,L (for example rw04 and I write rw04/-4) the fmc create discontinuity before rw04 and create waypoint after rw04 at 4 miles

If I have runway with R,C,L the fmc don't create any waypoint

 

 

Each item you listed should be a separate post... these issue are each quite different from the other.

Why do you think the FD should "disappear" at 50 ft?  Do you mean A/P disconnect or F/D turn off?  Neither will happen on approach at any altitude so I ask why do you think this is a problem?

There is no reason why you cannot change the SID or departure runway at the gate or at any time before takeoff, and after takeoff you could in theory change SID but that would be an unusual event.  You need to provide the steps you take to try to change SID, and no I'm not going to watch a movie.  Write it down please.

Creating a custom waypoint based on a runway fix requires that the navdata has a runway fix defined in the sidstar file.  Only a fraction of the terminal procedures have these defined and they are usually RNV approaches.  This just came up as a topic here a few days ago, and there are as many ways to do this as there are ways to skin a cat.

All his queries have been answered if the ticket opened.

1) FD has no landing flare capability. FD command bars retract from view at approximately 50ft RA during ILS approach. This has been confirmed to me by more real pilots of 737-800.
There are many videos 
min 3:02
https://www.youtube.com/watch?v=ag6rlU-CDwY&t=79s
min 14:47
https://www.youtube.com/watch?v=33gUSRqF5u8
min 6:35
https://www.youtube.com/watch?v=bOvmRd3tEfc&t=3s

I found a possible answer in the FCOM pg 11.31.38 where VNAV guidance is suspended when either GS becomes active or passing the runway waypoint; however this is an option and in any case does not answer to the FD guidance being suspended on a coupled approach at 50 ft. The FCOM section on AFDS Automatic Flight Approach and Landing in fact discusses how the FD will provide flare guidance on pg 4.20.19.

One has to be careful when using the internet to find videos of how the aircraft is supposed to behave and then purporting those videos to be authoritative.  I suggest that if you believe the PMDG is incorrectly modelling actual aircraft behavior for FMC U10.8 that you provide reference (other than non-attributable sources such as you tube videos).