Well, I've made a little progress. I completely uninstalled and reinstalled both Steam and Civ 5. I went to the Mods section and downloaded several mods, maps, civilizations and scenarios. I then installed them and enabled them. I went back to the mods section and started a single player game. Before the only two options were Custom Game and Load Game. Now I have a third option of Set Up Game. I check the Custom Game first as that is where the Mongol mod resides. Still no other mods are available. But, I now have the third option in the Set Up so I open that up instead. The same screen comes up as the main menu single player allowing me to choose various options.
However, when I click on the leader I can only see four leaders and the scroll bar on the right is gone. I can see a little part of the top of one leader below so obviously a glitch. If I click on the maps I do now have other options including the TSL maps, WW2 map, etc. I decide to go with the advanced setup and click on that below. Now I go to choose my civilization and I have the normal included ones but also all the other ones that were included in the downloads. I can also choose a different map.
Herein lies the new problem. If I say choose the Scottish civilization on a TSL large map and start it, everything seems to go fine initially. The game loads with a background for William Wallace. When the game shows as ready to begin my journey, I click on that to start. The game then closes with the game over option of going to the main menu or "playing just one more turn...". So, I tried another custom civilization and this time I got a CTD. Now what?