First you create a new field like described in the chapter Create and configure fields. Then you click on the tab Field triggers. You should now see the following:

You have three options what will happen if the field is changed, a value is written into the field or if the field is deleted: Activity Series, File or None.
Activity Series:
A series of activities that you created previously in Act! will start. By clicking on the three dots on the right you open a window where you can pick an activity series:

Go on OK.
File:

You can choose a program that will be executed or a link that will be opened. A click on the three dots in the right corner opens a window where you can choose a .exe-file, .bat-file, .com-file, .lnk-file or .url-file. Next, you go on Open.
None:
No action will be performed.