wkhtmltopdf-amd64
is a binary for amd64 platform computers. Unless you iMac has an AMD chip under the hood, that's why it's failing. You want the OS X binary here:
http://code.google.com/p/wkhtmltopdf/downloads/detail?name=wkhtmltopdf-0.9.9-OS-X.i368
Heroku must run on AMD64 or their virtualization is an AMD architecture. That should be why it works on Heroku.
Update: You may want the full app for OS X from here: http://code.google.com/p/wkhtmltopdf/downloads/detail?name=wkhtmltopdf.dmg
Not sure how you installed wkhtmltopdf, so don't know which is appropriate for your setup.
UPDATE!!!
If you download the wkhtmltopdf-0.9.9-OS-X.i368 binary, you can rename it to just wkhtmltopdf:
mv wkhtmltopdf-0.9.9-OS-X.i368 wkhtmltopdf
then you need to make it executable:
chmod 775 wkhtmltopdf
You can then test that it runs with:
./wkhtmltopdf
You should see the help/usage info.
If you downloaded the .dmg file, it is a packaged app, but to call it from a program you will have to give the path to the executable which is:
wkhtmltopdf.app/Contents/MacOS/wkhtmltopdf
That is a relative path (note no slash before the wkhtmltopdf.app) so treat accordingly. The .dmg version is at .11 vs .09 for the binary. Don't know the difference though, you'd have to check the docs.