Please help a non tech savvy bloke step by step how to play USA DVDs on uk macbookpro

Use the free VLC.

Region encoding is the mechanism that enables motion picture studios to control the worldwide release of their movies. It is required by the DVD Forum (http://www.dvdforum.org/forum.shtml) in all commercial hardware DVD players. Every DVD-Video disc contains one byte of data representing a region code, which limits where the disc can be played.

http://support.apple.com/kb/HT2397

Once you have set the region in DVD Player five times it cannot be changed again.

Instead, use VLC to view videos from different regions:

http://www.videolan.org/vlc/download-macosx.html

Or you could simply rip them using an app like handbrake, assuming you purchased them legally in the first place.

Copying for backup purposes is probably illegal in it's strictest sense, but truth be told its a bit of a grey area. In the UK at least the Govt. is brining in legislation while will allow personal copies of CDs / DVDs to be made legally.