Garry already said this and it's right on.
When you install make sure to select vbulletin integration and enter all the correct information. Basically you can not find the global.php file because you are not inputting the correct server path to the forum directory on the last step of install where it asks urls and paths for the integration. We make suggestions but you need to verify and correct. As long as everything is entered correctly on an install things go fine.
Here are the number 4 mistakes I see on vb3 installs so you can avoid them when correcting your issue.
1. Customers installs and enters forum server path incorrectly resulting in global.php not being found.
2. Customer enters in first step of install incorrect database info to connect to vbulletin. IE enters photopost's database info and then tries to do an install and gets mysql fetch array error line 25 of vb3.php
3. Customer does not enter mysql table prefix of vbulletin tables resulting in same error above mysql fetch array error line 25 of vb3.php.
4. Customer does not enter proper cookie prefix for vbulletin which results in admin login loop trying to get into photopost admin. We use the vbulletin cookies on front end since we load the vbulletin header however on admin side since we do not load vbulletin header etc you must have cookies properly configured.