- Create a new level
- Add your prefab and centre it
- Texture it
- Save as new file (.wmp)
- Click Build button and click "Build Map Entity" then OK (.wmb)

- Open your level
- Add Map entity and select the wmb file you've built previously
- Assign an action to it.

action makeTransp
{
my.transparent = on;
my.alpha = 50; // play this value
}

Finished.