If you're using the Graphics command (fullscreen) in Blitzplus, then you'll probably have to code your own routine with Keyhit. As Juniper said, Input doesn't work the same way with Blitzplus as it does with Blitz3D or BlitzBasic. I think, though, that someone has already done this -- check out the code archives. As an advantageous side effect, the rest of your game can still run even when it's getting user input (i.e. it doesn't stop running its main loop while the user enters input).
When you get more used to it, a more Blitzplus-ish way would be to use a full-screen canvas and do user input with textfields or something like that.
When you get more used to it, a more Blitzplus-ish way would be to use a full-screen canvas and do user input with textfields or something like that.