I think it's failing because Dropbox is user-centric and by default LANDesk packages are installed by Local System, which isn't like a typical user.
You can go into the package and click on Accounts. Then change to use "Current User" if the users have enough rights to install Dropbox this should work great.
If they don't, you can try using "Run as a specified user" and specify an administrator level account. However, I don't think this will work as Dropbox is tied into the user account and would likely only work if that specified account logged in, but it's worth a shot.