I've made many of those sweaters, in fact, I just finished one! On the last round of the body you had to bind off a certain number of stitches in two places for the underarms (mine was 12). When you get to the last round of the sleeves, you also bind off an identical numbr of stitches. Now to put it all together...
Knit the stitches for the front thn knit the stitches from the sleeve where the gap for the underarm is. Continue to knit the stitches for the back and then the stitches for the second sleeve. Hope this helps. Feel free to PM me if you need more help.
OTN: Mystic Water KAL
Felted Entrelac Tote
Finished: Socks for DS
On Deck: Beret from Gathering of Lace