How to select a Fund for a new opportunity
1. Leave the Fund blank and the default value will auto populate
If you do not fill in a Primary Fund when you create the Opportunity, or if you select a Campaign that doesn't have a default fund assigned...
Global default value
...the Opportunity will save with the global default value automatically selected.
Allocation
An Allocation for that Fund is automatically created as well on both the opportunity and payment and any already created payments.
2. Select a Default Fund for the Campaign
If a campaign has a Default Fund...

Selecting the Campaign
...when you select that campaign as the Opportunity's Primary Campaign Source...
Setting the Primary Fund
...it will automatically be selected as the Primary Fund (with related Allocation) when the record is saved.