Hi Nono,
If you use the high-level imaq functions, then the default channel on your PCI-1408 will be used. You can set the default channel in Measurement and Automation Explorer(MAX) by right-clicking on a channel and selecting 'Set As Default'.
Or else, you can set the channel to acquire with imgSetBufferElement() with IMG_BUFF_CHANNEL as the itemType and the channel number as the itemValue.