im not saddle club girl but u cant do anything untill she looks at it and it pops up on
the screen

this happened to me aswell. save the game into a different slot, then reopen it. do this
several times and eventually she will open the letter. its just a glitchy game is all :)

im am terribly sorry i was not able to get this in April although i will still help you
even though someone has allready helped you. After you got the letter did you move or
click on anything? if you did then thats the problem. go to the menu, click on the save a
quite button, then go back and play on your slot. you will be able to replay the scene
where arthur give you the letter. DO NOT TOUCH OR DO ANYTHING! wait for a couple seconds
and if your horse is not blocking your view, then you will see your character, lisa,
carol, or steve, openeing the letter. you will be able to see the letter and after you are
done reading the letter and you clicked on the spacebar, you are ready to go!
Hope this helps!
Saddle Club Girl

you need to wait dont do anything when you get the letter then it will pop up on the
screen then you resd it hope it helps