This article provides information on how to enable Donors to make online pledges when setting up a new fund.
If a Fund has been enabled to allow Pledges to be made online, a Donor can make a Pledge to a Fund from the public site.
When Adding an Online Fund, to enable the ability for Donors to see a Pledge button and make Pledges from the public site, the box titled Allow pledges to be made online should be selected.
Check this out for more information about Pledges.