The only way that section may be hidden is if you are using the Opportunity > Products.
When "Opportunity Products" is checked one opportunity will be created for all items in the basket and each item will be set as a product. Because of this the primary campaign cannot be set.
In reviewing your case we also noticed that you are in an extremely old release of our application. Your instance is about 6 releases behind. We can upgrade your instance to release 9.1xxx if you want - all you have to do is to grant us login access and verify your Salesforce Organization ID and we will upgrade you for free.
Just let us know if we can be of any further assistance.
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