In previous versions you would use the --backup flag. In the current version they changed it so you don't need to specify a flag, just put the info in the box. You can use multiple backups just by separating them with spaces.
How to use a backup pool aka fallback support:
Fallback support is in the OpenCL miner only. In the flags, specify the info of the backup pool or pools. For example if I want to fallback to my account on slush's pool, I would write:
Kiv.GPU:mypass@api.bitcoin.cz:8332
When you do this do you pass a flag? In other posts it looks like --backup= is used. Do we to specify that in the flags? as in --backup=Kiv.GPU:mypass@api.bitcoin.cz:8332