It isn't boolean
If you have the proper version of Bryce...
First thing I would do is to make the cityscape positive.
Select the object then click the little A and choose positive
Next create a sphere..any primitive really...and hover it over your city scape
Select the sphere, click A and choose negative
Now, select both the sphere AND the cityscape and click G to goup them.
You should have the G change to a U (ungroup) and a letter C (cut) should also appear
Click the C
go to FILE and the Export Object option should now be available!

That's the long version of what Bryster explained up there....