How to activate field based on choice of dropdown?

I think I know what you're asking for.

You can place this into the Actions tab in your dropdown properties.

On Blur>Run A JavaScript;

var fldText1 = this.getField("Textfield");
var fldDropdown = this.getField("Dropdown").value;
if (fldDropdown == "Amendment") { //If dropdown selection is "Amendment" option
     fldText1.display = display.visible; //Sets the text field to visible
}
else { //if anything but "Amendment"
     fldText1.display = display.hidden; //Sets the text field to hidden
}

You can also set the value to read only as well if you don't wish the field to be hidden yet still be unable to be edited.

var fldText1 = this.getField("Textfield");
var fldDropdown = this.getField("Dropdown").value;
if (fldDropdown == "Amendment") { //If dropdown selection is "Amendment" option
     fldText1.readonly = false; //Sets the text field to editable
}
else { //if anything but "Amendment"
     fldText1.readonly = true; //Sets the text field to readonly
}

If you set the text field to Read Only by default in its properties you should be good to go with that.

Hope this answers your question.