Classic runs a lot of programs (bless its heart <3), but it's not perfect, so there are some pesky programs that it won't run. It might unfortunately be that Poser 1.0 is one of them. (There are some awesome older games I've wanted to play that don't work, like Museum Madness and Zoop.

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The fastest way to figure out if that's the problem would be if you have a computer running 9.0 or earlier natively to try it out on, but I realize that that's not particularly a common thing these days to have lying around. Otherwise, you might try running it on BasiliskII (a 68K Mac emulator) or SheepShaver (a PowerPC Mac emulator), but you'll have to have a copy of an older Mac OS ROM and OS handy.
How ya doing, buddy?