I'm guessing you need help on holding back the enemies at the end by the ferris wheel.  In that case, the best tactic  is to plant all your C4 on the rise behind the Ferris Wheel (because many enemies go prone there and they are hard to see because of the grass), hide behind the bumper car ride, and when the helicopter arrives detonate the C4 and sprint to MacMillan. Before picking him up throw a flashbang to your right to blind any enemies attacking you from the Ferris Wheel.

Also, be sure not to plant any claymores too close to the C4 blocks, because when they go off they will take the C4 with them.One easy way to get through this level is hide behind the tree on the right of Macmillan , you can dodge a lot of gunshots there since the tree is thick enough, occasionally there will be one or two guys climbing up the stairs of the ferris wheel or approach from your far right side , but only one at a time, so it's fairly easy, just watch out for any grenades they don't often throw at you. 

Another easy way to get through the end of the level, is to plant C4's and claymores around the ticket booth and then hide as far back as you can in the corner in there. Close-by grenades (like at the doorway) won't kill you and some rare grenades that make inside will be easy to throw back. Just sit back and relax until the helicopter comes.

After placing Captain MacMillan behind the ferris wheel, a good place to hide is in the opposite corner of the map, where you can crouch in a corner behind an apartment building. It is inadvisable to stay there for too long, as you could easily become surrounded, or trapped by grenades. When you get a good opportunity, run towards an alley next to the building with the pool. This alley is where the enemy would normally spawn, but they won't as long as you're there; they'll spawn from other places. This alley is a great place to take cover, as the enemy can only attack you from one direction, and MacMillan will kill a lot of enemies before they come to close to you.

If you do go into the alley, make sure you can get to Captain MacMillan in time, because you will be very far from him. When the helicopter comes within 1000 meters, it's time to get moving. Stay cautious, as the last checkpoint was when the helicopter was around 6000 meters away, but don't be afraid to rush towards MacMillan. Once you've picked up MacMillan, your team should be able to hold off any enemy fire, and the mission should be complete.

Yeah, I got a lot of this info from callofduty.wikia.com.  It helped me on this mission too.