MyCloud doesn’t use a PowerPC architecture processor; it uses an ARM architecture. So you have downloaded the wrong image.
You might try scrolling to the bottom of the downloads list, and using:
http://www.twonkyforum.com/downloads/8.2.1/twonky-WD_MyCloud_04.xx.xx-arm-8.2.1.zip
or the v2 or v3 images, as appropriate for your firmware.
I’d be interested in how you get on, and if you have any licence issues; I have never tried upgrading ( note the twonkyforum comment “For all versions – PC as well as embedded – you will need to purchase a license from our online store to run them beyond the trial period”). There’s some discussion of where twonky should be placed, and how to restart it here:
To sort out your mess, you could downgrade your firmware, then upgrade it again: search the forum for ‘downgrade’. You might even be able to get it to simply re-install the existing software by using the downgrade trick of changing the version string.
