Eject Windows 7 Installation DVD 1, during boot camp install on MacBook Pro (Intel Based)
In short, you can't eject a CD/DVD during Boot Camp install. You can only eject it:


during boot (holding down the trackpad button)
while running OS X or recovery
while running Windows with Boot Camp drivers installed


The workaround is to copy the contents of the second DVD to a flash stick, insert that into an USB port BEFORE installation begin and, when asked for the second DVD, provide the path to the flash stick.