I feel like I've been bashed, as in knocked around. Between the TSL2561 data sheet and the info on the i2c commands, I came to this: Data sheet p. 13 gives the address of the control register as 0h. (Sounded funny to me but I'm an ignorant beginner) Then p. 18 tells me to access this register and write the value 03h to it to power up the device. Back to p. 13 it tells me that should I do this, I can read back that 03h value to "verify that the device is communicating properly."
So for the i2cset command I write, i2cset -r 1 0x29 0x0h 0x03h following (I think) the requirements; -r option to read it back, 1 is the i2c bus, 0x29 is the chip address on the bus, 0x0h is the register address the data sheet gives me, and 0x03h is the value I want to write to that register.
Result is Error: data address invalid! Wants something between 0x03 and 0x77.
Hex is supposed to be within the range of 0-f, isn't it? So I'm missing something really basic.