I used my "account no." (ex: M1234567) because that's verbatim what is written on the BitInstant site. What they're really looking for is the EXCHANGE ACCOUNT, which is a 6-digit number or your MtGox username. I guess this is the reason why I still don't have my money, and it frustrates me.
FYI if you don't mind spending a bit more you can buy bitcoins on ebay. There are a few reputable sellers that will have coins to your btc address within 1 hour.
It seems crazy, that if true, this isn't made clear on the site! I've previously used the account number displayed on the Mt. Gox homepage more times than I care to count, so if now something has changed it amazes me they (BitInstant) have not provided clarification.
I guess for now I'll continue to use my MP to BTC guy for 5% ...