Can't find the preference menu

[Katalyst777]

Preferences are only available in Pro, as is the Convert to Terrain Perimeter option that you are referencing.

Instead, you're going to have to create a Terrain Perimeter (if there isn't one already in your plan) by selecting Terrain-> Create Terrain Perimeter (if its greyed out than you probably already have one) and then use the edit handles and Change Line/Arc and Break Line edit tools to resize and reshape the terrain perimeter.

If you already have created the outline with CAD lines, it should be pretty easy to reshape the terrain perimeter to match that.