Well, it depends which version of ACT! you have, but you'll almost certainly need the ACT! SDK.
That can be downloaded from here: ACT! SDK Download
There's a good post on the ACT! developer forum that has a .zip file attachment. The attachment contains some developer training materials, posted by a Sage employee, and within that material is a complete example for manipulating contact data.
From the included "readme":
Insights2007ACTSDKTraining.zip
contains three files:
ACT! SDK Training.pdf is the
presentation that was used for the
session.
Samples.zip contains a contact-bound
control demonstrating entity data
binding and design-time capabilities.
Act.DevNet.Extensibility.Plugin.zip is
the pluin template. Copy it to your
Visual Studio
2005\Templates\ProjectTemplates
directory. It will then be accessible
via File->New Project, Select Visual
C#, under My Templates.
The link for this is: ACT! SDK Training Session Material