From what you describe you missed the barrel lug with the slide stop when assembling it. Pull the barrel as far forward as you can and then try to move the slide to align the take down notch. The slide stop can only be removed when the slide is in the correct position. You may need to use a rubber or plastic mallet to move the slide. If this is the case be careful, you don't want to damage anything, just tap the slide.
Let me know how you make out,
I will be out for the next 3 hours, I will check in when I get back.