i don't know. i guess you have to write some sort of function and pick data from cell,where the last price is.
if you using API it's pretty easy, but for dde-have no idea.
there is couple groups on yahoo,related to trading thru ib api and excel. you may ask same question there- http://finance.groups.yahoo.com/group/TWSAPI/
from my own expirience-pick up open price not an easy task and in many cases,price that you pick, will be far away from "official" open price,which will be relased later. if you doing something like backtesting of the system,where entry @ open price is involved-i suggest you to think twise, before you start trade it. talking about nasdaq here.