Creative writing - two sorts.

K is back to writing her book.  I am still not allowed to read any of it, but she has now shown me some illustrations that she has done for it.

M on the other hand did creative writing of a different sort recently.  She gave me this letter.

She expected a reply, so I wrote her a reply taking the letter at face value, to which she wrote back to me just five words 'You got it all wrong.'  With some help, I then realised that it was in code and once I had worked it out, I acted accordingly.  To find out what it said and why, highlight the following:
The capital letters spelled out :  Meet me in Granny's room at 3pm.