and I already make sure that connected USB cable is correct...
(USB mini A(5P) Male to USB A Male.. right?)
(it's directly connected between Beagle & Keyboard)
BeagleBoard OTG USB must be in host mode to talk to your keyboard.
I have found that a good way to check that the BeagleBoard OTG USB is
in host mode is to check the voltage level at the OTG connector pin 1
with a voltmeter. R5 is a good place to probe it. You can get ground
at OTG connector pin 5. If BeagleBoard OTG is in host mode, it tries
to set pin 1 to 5V. If BeagleBoard OTG is in device mode, pin 1 is
set by whatever is plugged into the OTG port, which would be 0 for an
unpowered device like a keyboard.
There are lots of USB OTG suggestions in the BeagleBoard Wiki http://elinux.org/BeagleBoard#OTG and BeagleBoard FAQ http://elinux.org/BeagleBoardFAQ#USB_OTG_connection_.231. These
include hardwiring USB OTG connector pins 4 and 5 together at the
BeagleBoard, giving a Linux command, and modifying a USB Mini B cable
into a USB Mini A cable. The general problem is that USB Mini A
cables and adapters are hard to find, unlike Mini B cables which are
included with lots of USB devices.
An easy way to make sure this is really your problem is to short USB
OTG connector pins 4 and 5 together with a probe while booting Linux.
Once Linux is running, you can take the probe away -- at least that's
how it works with Ångström.
I made similar mistake when I bought a usb mini-a cable at Yongsan, the biggest off-line electronics market in Korea. They said it was a mini-A cable but I figured out that the pin 4 and 5 were not connected.
As John explained well, the easiest and quickest way is to short the pin 4 and 5. It needs just one second.
However, then you cannot take advantage the future possibility that you could connect the BB to your pc and have the board work as a usb device. (I don’t know if you really want to do this now or in the near future, though… :-))
So, what I did was… I glued a micro-switch on top of the stereo socket and threw two wires to the pin 4 and 5 of the mini-USB socket. It looks a bit messy but works perfect.