dd if=/dev/mmcblk0p1 of=/mnt/us/mmcblk0p1.img bs=4K
for if= (input file) and of= (output file) you need the full pathname and filename.
Busybox help messages are brief reminders intended for the knowledgeable user.
Let me guess, you are not a Linux (or MacOSx) user.
"Hack is just a four letter word." - - With apologies to B. Dylan