From the Scrivener File menu, choose "Compile".
Put checkmarks next to what you want to send. I send chapters, not character notes etc.
Select "Compile for ePub."
Click on "Compile"
In the Export screen that pops-up, make sure the Folder is set to the Dropbox folder that your phone is linked to.
Click on Export.
On your phone, open your Dropbox app.
Click on the file in the dropbox.
When you get the "Couldn't Load File" message, click on the "Send to" icon in the lower left corner.
In the pop-up, choose "Open In . . ."
In the next screen, choose "Open in iBooks"