(Thanks for the command line
chmod a+x mkamsboot)
Maybe the problem was the USB mode (settings/usb mode, in the "Sansa clip+" settings)
Default option is "automatic".
Before installing files, You have to change this option to "MSC" in the "Sansa clip+""USB mode" (instead of "Automatic or "MTP").
I bought my Sansa Clip+ yesterday, and with the user's guide and this post

, rockbox works today

it's worth it, it's a wonderful software, thx

PS : Sorry for my English, if I left some mistakes...