You need to get the actual ID values from your SOPC system. If you look in your quartus project folder, you should find the datasheet for your SOPC system. The file has the same name than the SOPC project, but with an html extension.
There you'll see a box model of the SOPC system, with a list of peripherals. Look for the one that's f the type altera_avalon_sysid and remember its name. In my case it's sysid.
Just underneath you have the address map. Find the address for the sysid component, and this value is the one that you must provide with the sidp argument to nios2-flash-programmer.
Then scroll down to the description of the sysid component, and there you'll find the id and timestamp values that you must provide to nios2-flash-programmer.
And as said Jake, ensure that the same SOPC system is currently running in the fpga before you run nios2-flash-programmer