Simulation of an Operational Transconductance Amplifier in Orcad Capture
how can I connect it to power supply (VDD, VSS)?


You can't and you don't need to. In your first schematic you're using an ideal voltage to current converter. Such a component doesn't have supply, it is "its own source".

What that means is that unlike in a real circuit (using transistors etc) this ideal OTA has no issue with supplying an output voltage of 1 million volts if that is what it takes to make the current flow. Just be aware of that. So you need to make sure that the output current of the OTA can "go somewhere". If it can't, the ideal voltage to current converter will simply apply as much voltage as is needed.

What might be possible is to limit the output voltage that the ideal voltage to current converter will output. Not all simulators / symbols support this though.

You have drawn the supply batteries inside the first schematic. That's not logical, if you buy an OTA chip then the batteries aren't in the chip either.

Move the batteries to the 2nd schematic, this is the "testbench" schematic.

To make a hierarchical schematic, make a symbol from the first schematic (the one with the ideal voltage to current converter). Include that symbol into the testbench circuit.

I can't judge if the guide you used is any good, I do suggest looking at other guides and tutorials (search for "orcad hierarchical design") as well and simply do exactly the same. Once you have something working it is much easier to change it to how you want it and see if that works (or not).