I downoaded some of the roman units and when I get a leader my CIV crashes. How do I resolve this problem?

Can you post a screenshot of what's happening? What's the error message? Is it when a Great Leader spawns? What version of Civ3 are you using? (that is, vanilla Civ3, PTW, or C3C, and the version number, like 1.07, 1.29f, 1.04, 1.27f, etc.)

Make sure you're updating your pediaicons.txt file. You might want to look in the Customization and Creation subforum Tutorial section for a tutorial on how to add units and stuff to civ3.