Go to a member record and then click on Profile access settings. You would see that Last name is set to No access. Click on Edit button and change to Anybody or Members, as you require.
See for more details: http://help.wildapricot.com/display/DOC/View+and+edit+member+record
Also, you can do this change on mass scale (for all records) by using Export-Import.
See: http://help.wildapricot.com/display/DOC/Export+members+list and http://help.wildapricot.com/display/DOC/Import+Guide
Denys Zlobin
Helpful Apricot
http://www.wildapricot.com