Selecting an Option in "Choose an Item" on a Mac

If you're going to use Dropbox, don't copy the content of your browser's address bar. Go to the Files page of your Dropbox account, point to the file you want to share, then click the Share button that appears at the right of that row. In the share window click Create a link, then click Copy link. Copy that link then paste it into a message here.

It's also important that you specify your version & update level of both Office & OS X... correct responses often are dependent on that information. Additionally, when you submit a question please use the Filters on the left side of the Community web page to select the appropriate Office Product rather than Unknown/other. If you aren't sure, in the program click its name in the main menu then select the About listing [e.g., Word> About Word].

FWIW with no other info to go on, my guess is that if you're the form was created using ActiveX Controls which are useless on Macs. ActiveX is totally unsupported. Another possibility is that you're using a version of Mac Office earlier than Office 2016 & either ActiveX or Content Controls were used in the form... neither type are functional in Office 2011 or earlier.