Open up your Start Menu and navigate to Administrative Tools -> DNS.
Expand the ServerName (eg: sv2400), then 'Forward Lookup Zones'. Click on the domain you wish to edit, then right-click in the right-hand window pane and select 'Other New Records...'
Scroll down to 'Text' and select 'Create New Record'
Leave the 'Record Name' blank, and enter the following in the 'Text:' field:
v=spf1 a mx -all
Click OK.
Repeat steps 4-8 for each domain you wish to add SPF records to.
Note: DNS record changes will take about 4-24 hours to propagate throughout the internet.