Below is a step by step guide to the Nod missions 14-16

Mission 14
Take your spy across the bridge then towards the north west corner, you will see a very small base, wait until the spotlight turns away and run to the building.
You will see a demonstration of the Mammoth MK II
Move your MCV and tanks across the bridge, use a reckon bike to scout to your left.
You will see your first patch of Tiberian. Deploy your MCV near the Tiberian.
You are quite venerable this side of the bridge but you need the space and the Tiberian.
Build a laser tower and a cyborg then build a mobile repair vehicle. Keep this hidden and keep your tanks on full health. Once you have a nice base build laser wall round it. And an obelisk of light by the entrance plus a laser tower. This with your 3 tick tanks is enough to fight of attacks.

The Mammoth MK II is almost in the north east corner. Use a reckon bike to scout out trying to stay to the north edge. There is a small section of wall you can blow up.
Once you have found the base all you need is 4 pieces of artillery (more to make it quicker) and you can take out the base! The units only attack you one at a time as they are guarding the mammoth which mean that you can kill them before they get to you.
For extra security you may want to use some of the new air power at your disposal but keep it defending as they have a lot of SAM equipment.
Easily done!

Mission 15
Put all your men into APCs. Ok, keep to the southern edge as much as possible.
You mission is not to get the base at all! You have to make your way up to the northeastern base where the guy is. Then get the guy back to were you started!
Get your APCs up near the top base. Save game! Get 2 APCs; in one fill it with engineers and the other needs to have a Toxin dude in! Run the apcs through the defences. They will get blown up. Kill the light infantry with your toxin guy then you will get command of the mutant. Run him outside the base where there should be an APC (if still alive), 2 toxin guys and a rocket launcher. Run them all together back to your base you will have lots of people followed you! But you should make it.

That is how to complete the mission, to get a higher effiency you may want to use your engineers for something other than cannon fodder! Like capturing the base as you were asked to, but if you try and do that you may lose some of your men which are needed to complete the mission.
It is your choice!
GL!

Mission 16 bottom, Illegal Data Transfer (Ion Curtain codes)
Scout out with your spy, you will see 2 towers.
Go on the ridge and shoot at the first one with rocket infantry.
A few light infantry will try to kill your racketeers kill them with your light infantry.
Follow the path to the right with your spy, there will be some troops, run your spy away from them and kill them if they attack you. When there is a patrol wait for it to pass to do not attack it.
Ok at the end of the path you will see your first upgrade centre to steel the codes from. The second one is back near were you started.
It on has a small sandbagged entrance with one light infantry sanding by. Use all your men to kill the light infantry. Now you can get the second set of codes.
Scout with your spy but not on the path but within view of it. You will see there is only one other path you have taken. This goes up a level; however, at the top there is a Vulcan cannon. However, there is a section of wall, which can be blown up to the east of this. There is a patrol covering the area so take care. Use one light infantry to quickly take out the "secret entrance" and them reunite him with the rest of your squad.
Get the 3rd and final code with your spy and them take him to the APC, which arrives in the south east for him.