Since I know that the menu structure of a TI84 is more than chaotic I've mentioned in my previous post where you can find the menu items to build the necessary formula:
sum( := List Math 5: means: You'll find the sum( command under LIST (=2ND STAT) --> MATH --> 5:sum( := List Math 5:
seq( := List Ops 5:
List := 2nd Stat