You have to grab the little tab on the right side of the running-avg node and expand it so there are 15 dummy pins if you want avg of 15 values. The dummy pin is not used (hence the name). Giving it a value does nothing. It exists only to take advantage of the variadic function of XOD.
Running-avg-AVG should have output immediately on startup. Since values initialize to zero, the initial AVG will be 0. After 200ms, it will average 14 zeros and the new value. After 15 x 200ms, you will start getting valid data (15 samples averaged with no default zeros), which will update every 200ms with the average of the last 15 values. It is a running average. Every time a new value is read, it drops the oldest value & averages the last 15 samples.
As cesar said, there are examples in the library for each of the nodes in the library (utils-example-avg for the running-avg example).