The PFI lines on the 6133 at system power-up are configured as high-impedance inputs with a 50 Kohm pull-up resistor which is why you read +5V at PFI0. You can find this information in the specifications section of the S Series Help.
You should have no problem connecting your digital trigger signal to PFI0 as long as it adheres to the digital logic levels found in the digital I/O section of the specifications document.
Have you tried using an example that uses a trigger yet? I would recommend connecting your trigger signal to PFI0 and then running a shipping example found in LabVIEW at Help >> Find Examples. If you are using NI-DAQmx (using this driver is strongly encouraged) then you could run the example Acq&Graph Voltage - Int Clk - Dig Start found at Hardware Input and Output >> DAQmx >> Analog Measurements >> Voltage. If you are using Traditional DAQ, then you could run the example Acquire N Scans - Digital Trig found at Hardware Input and Output >> Traditional DAQ >> Analog Input >> General