ClintonWestwood

Hi there. I'm pretty new to Kerbal and find it is a great bit of software. I'm have Mechjeb installed and I'm aware I have to place the Mechjeb part on my craft. Once that's done, how do I activate Mechjeb. I'm certain I've seen the likes of Scott Manley use it whilst building the rocket to determine things like Delta-V and other useful stuff. How do I do this? Is it Mechjeb or another mod? Thanks