I bought the Xbox One, Forza 5 and the 12 Months of X-Box Live card with the free McLaren. I have done all updates and I have checked “add ons” and everything shows as installed. The “free” McLaren is available to buy for 280,000 but was I not supposed to get that car for free?
No,you need to buy with in game credits.
It sounds like what you’re describing is the Forza Motorsport McLaren MP4-12C, described here:
http://forums.forza.net/turn10_postsm15107_FM5-Cars-and-DLC-list.aspx#post_15107
If you installed the DLC correctly, you should see two versions of the MP4-12C in the Buy and Rent cars list: one in factory paint, and one upgraded to A700 with the special livery. Installing the DLC does not gift the car to your garage - you’ll need to buy it with credits or tokens if you want to drive it in Career or Rivals.