The only times I've had the LiveCD ask for a username/password have been when it's a bad burn, or when it's a low-spec PC.
Try entering the username as root and leaving the password blank
GA-965-DS3, Core 2 Quad 2.4GHz, 8GB DDR2, 2x1TB HDD, Linux Mint 11
Acer Aspire 5332, Pentium dual core 2.2GHz, 4GB DDR3, 500GB HDD, Linux Mint 11, Linux Mint Debian
Compaq CQ-60, Sempron 2.10GHz, 4GB DDR2, 120GB HDD, Tinycore 3.7rc2 (Minecraft server!)