I previously owned these 2 airports for FSX and had no problems, but I tried copying these to my P3D folder and install them manually, but they are both missing terminal buildings. I've seen youtube videos of people flying in and to these airports in P3D before and the terminal buildings shows up fine for them. I'm not sure how to fix this.

 

I've managed to copy other FSX airports over manually without any problems except for these 2. They are both Thai Creation - VVTS and VVNB... Tan Son Nhat Airport, Ho Chi Minh City, Vietnam and Noi Ba International, Hanoi, Vietnam.

have you managed to solve the problem, if yes then how?

Uh, as far as I know both of these airports from Thai Creation are only for FS9. So there is your problem.  You may have gotten it to work previously in FSX via some hacks, but those are no longer working with P3Dv4.  It also sound like you got those hacks from someone else too, so you can try contact those folks.  Your best bet is to contact Thai Creation, but good luck with that.  They have not even released an official FSX version, let alone creating one for P3D.  I know because I tried to get in touch with them, as I own both of these for FS9.  When asked, the developer said that there are too many compatibility issues going from FS9 to FSX.