If the X10 dimmers are added as an X10 dimmer in MobiLinc, then if you tap the gear icon (MobiLinc HD on the iPad) you should be presented with ON, OFF, DIM, BRIGHT options. If on the iPhone tap the your device's row under My Devices to see the same dimming options.
The main switch graphic will only do ON/OFF due to the lack of direct percent Dim commands that one finds with INSTEON. For example, in INSTEON you can tell the dimming device to go to 43% on. in X10 you can't explicitly command a device to come on or to a specific ON level. All you can do is issue Bright/Dim commands over and over again until the device it at the level you'd like it to be.