iOS: Error adding external framework to my project

I'm trying to use the Unrar4iOS framework in my project. I built the Unrar4iOS project, copied the framework's folder to my project and ad开发者_运维问答ded it to the project's existing frameworks. I have my Framework search paths all set. But when I compile I get this error:

 cd /Users/alex/Documents/iphone/production/iphone/myapp
    setenv MACOSX_DEPLOYMENT_TARGET 10.6
    setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/g++-4.2 -arch i386 -isysroot /Developer/Platforms/

ld: framework not found Unrar4iOS
Command /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/g++-4.2 failed with exit code 1

What do you think?

Upgrading to Xcode 4.2 and building on iOS 5 resolved the issue... Don't really know why...