On default BlogEngine.Net 2.5 installation, to use ReCaptcha, you can follow the default instruction here:
Sure! But apparently that's not all. It still doesn't appear in my page. Apparently there are 2 more steps that you need to do:
1. Go to you project folder > \App_Code\Extensions
You'll see ReCaptcha "folder" there. You need to move all the .cs files from that folder, up 1 level, to the "\App_Code\Extensions" folder itself.
2. Go to your BlogEngine.Net Dashboard > Extensions panel. If "SimpleCaptcha" is enabled, you'll need to disabled it.
Hope it works for you.