Thank you for contacting us. You can download mSecure from the App Store app on your iPhone first. To do that, open the App Store app, then search for mSecure, and you should you shouldn't have any problems downloading the app.
After that, since you are using Dropbox syncing, you'll be asked if you would like to link mSecure on yoru iPhone to your Dropbox account. Follow those instructions, and after mSecure is linked, your data should sync over from the Android's mSecure app.