Are you trying to create an instance for each item in thisFifo, or are you trying to create one instance that can switch between all the different items in thisFifo depending on which one you want to work with?
What exactly are you trying to do with these tags? There is probably a better way to go about what you are doing. If this is an actual FIFO, I find it strange to read specific positions within the stack.
I got the whole path by right-click -> Copy Path on the UDT instance in the tag browser.
You may need cast your integer to a string if you're going to use the value in an OPC path. I agree with @jlandwerlen that this isn't an intuitive design choice to me, but you're the expert on your application.