Terry,
There are two good access points. There's a parking lot at Catoctin Furnace on Catoctin Furnace Road (right turn off of Rt 15). From there the stream is in Cunningham Falls State Park for 0.8 miles until you hit private land. You can also park in the state park at the Manor Area lot (left off of Rt 15). There are spots by the stream if you drive down the hill past the visitor center/aviary. That's a few hundred yards upstream of Catoctin Furnace, but still well within the park. The upper limit of that public section isn't marked or posted so it's pretty easy to wander onto private land.
There's also another public section further upstream off Catoctin Hollow Road, which starts after the small bridge over the stream past Trout Run, but ownership and access is patchier up there.
I'm curious where you were when you were asked to leave.
Hope that helps.
Seth Denbo