Actually, the class-tgm-plugin-activation.php is found in /themes/kleo/kleo-framework/lib RATHER than /themes/kleo/lib.
I tried that on my system as follows:
1. Downgraded to kleo 2.3.4.
2. Activated the pmpro bbpress plugin.
3. Upgraded from kleo 2.3.4 to 3.0. At that point, the admin login produced a 500 error and I couldn’t get in.
4. Renamed /plugins/pmpro-bbpress/class-tgm-plugin-activation.php to /plugins/pmpro-bbpress/class-tgm-plugin-activation-ole.php
5. Copied /themes/kleo/kleo-framework/lib/class-tgm-plugin-activation.php to /plugins/pmpro-bbpress/
6. Logged in as admin using Kleo 3.0 and it works!
This is a good sort-term fix. However, it appears that pmpro bbpress needs to be updated to fix this problem. I have posted a companion trouble ticked in the pmpro forms at:
http://www.paidmembershipspro.com/forums/topic/problems-activating-pmpro-bbpress-addon-version-1-2-in-kleo-theme-version-3-0/#post-31767
I have asked them to update their plugin.