Creating (and accessing) an array in MIPS

The error is telling you you can't put data (.word 3, 2) in the code segment. "Text segment" is an old style term meaning "code segment" http://en.wikipedia.org/wiki/Code_segment

The assembler wants you to declare a data segment and put the array there. I've never done Mips assembler, but I would expect it to be something like this

.data
list: .word 3, 2, 1, 0, 1, 2

.text
start:
li $t0, 0x00000000  #initialize a loop counter to $t0
li $t4, 0x00000005  #last index of array
li $t3, 0x00000000  #this will hold our final sum
la $t1, list  #the address o

You have to put this line:

list: .word 3, 2, 1, 0, 1, 2

Into the .data section. Check this quick tutorial.