ahhh.. sorry the confusion is because there are in effect 2 tests that can be performed.
The first one is the BTw performance tester (which gives you your IP profile) and for that you use username@plusdsl.net into the website page where asked.
If that test is below par, you are offered to bypass the ISP network and do a test on the BTw speedtest domain...
For this you have to physically change the login in your router to:
user: - speedtest@speedtest_domain
password:- (this can be left blank but since some routers wont accept a blank field so you can put in anything such as test or even keep your ISP passy).