Answer
Use the "create new" menu to enter a new gift from the donor. Use a cash sale to enter a donation. Use a sales order to enter a pledge.
Context
In NetSuite, the donor is represented by a customer record.
The gift is represented by either a cash sale (for donations) or sales order (for pledges) transaction record.
NetSuite provides multiple options for initiating actions from the customer record.
Since you can rename transactions in NetSuite, it often makes sense to rename "cash sale" to "donation" and "sales order" to "pledge"