SPI Register read/write blocks are meant for memory devices such as EEPROM.
So in your case, following the example,
try using "SPI Master Transfer" block with a constant block having value [1, bin2dec('10000000'), 0] as input. Here in the binary value '10000000', the first 4 bits '1000' represents single-ended voltage measurement at CH0 is requested (refer to the datasheet for more info). To read from CH1, then '1001' should be used. To retrieve the voltage value from the output, say 'data', you have to model the logic mentioned in the above doc (also mentioned below) using Simulink blocks.
highbits = bitand(data(2), bin2dec('11'));
voltage = double(bitor(bitshift(highbits, 8), data(3)));
voltage = (3.3/1024) * voltage;