If this is your first visit to the Necropolis, you should have a quest called "Portal Decay" from Lazlo that tells you to enter the Necropolis "through the double doors in the Plaza of Conquests". Check your quest log file to see if you have a quest telling you to Explore the Necropolis. If you do, click on the Information Bubble in the upper corner of that quest box, and it will tell you what you still have to do.
Go back to The Plaza of Conquests, and look at your map. You can see the double doors at the top of the map, and they lead you to the Necropolis.
Also, Chase gave you a link with all of the quests listed for Dragonspyre. On that link you can click on any of the quests for more details (like "who" gives the quest, and prequests or requirements, etc.). Find the quest "Portal Decay" on that link and see what you are missing (prequests, etc.).