Hi @marty84
Yes you can use it within another plugin. If you’re looking to sell/distribute that plugin please have a look at: http://www.advancedcustomfields.com/resources/including-acf-in-a-plugin-theme/
As for your code try changing the_field to get_field. You’re attempting to save the value in a variable and thus should use get_field since that will return the value rather than the_field that echoes the value.
As a sidenote please try to add all code snippets inside code tags 🙂 You can see the code tag button in the toolbar above the textinput field.
