To define the channel when using pl1000GetSingle simply use the channel number e.g. channel = 1. PL1000_INPUTS is an enumeration but you can use the numerical equivalent. Enumerations can be accessed if a prototype file is used but it might be easier for you to use the former approach.
It might be worth looking at the C code example in the SDK to help understand the sequence of function calls. Also, one of the other forum threads should give you some more guidance:
The scaleToMv parameter was removed from the function arguments but not from the function implementation - you only need to use the following code for converting the raw ADC counts to milliVolts:
function millivolts = pl1000adc2mv( raw, maxValue )
%PL1000ADC2MV Converts milliVolt value to ADC count for PicoLog 1000 series
% raw - raw ADC value
% maxValue - maximum ADC count of the device
millivolts = (double(raw) * 2500 / maxValue);
end