Depending on how you're using Mailchimp, you might have members without corresponding contacts in your Act! database, because maybe they subscribed to a newsletter of yours on your Web site.
Mailchimp Plugin III for Act! allows you to easily import those members into Act!. Go to Import > Import Members from Mailchimp and follow the wizard which should be self-explanatory:
The members can be imported into Act! in a group (either a new group or a group of your choice). The purpose of this is to give you an easy way to track them once imported.
Importing multiple times will not result in duplicate contacts. Since Mailchimp unique key is the e-mail address, the add-on uses the Act! e-mail address field to check if a contact already exists. Note that if you have several e-mail address fields, the address field used by the add-on for duplicate checking is the e-mail address field that you mapped to the Mailchimp e-mail address field under Options>Field Maps....
Field Mapping is key when importing, so make sure you define your field map properly under Options>Field Maps....