how can I get into download mode of Cube Talk7x (U51GT-W)?
Download mode is a Samsung exclusive partition that only exists on these devices to allow Manual ROM installations from The ODIN PC client. Other devices do not use the download mode you are talking about, including the device you are using this method on.

To elaborate,

ODINs a samsung-only partition that allows form the installation of ROMS and bootloader updates via PC. It can serve Installations to many, many devices at once via bulk download (Which can support 50 at a time, I think. Of course, you probably do not have 50 USB ports) It is primarily used by Samsung when flashing new ROMS for shipping (the software was made by Samsung, after all) and Samsung users who want to root their phones and tablets in a "technical" way (even though the hard work is done by the the Root package Devs, mainly Yabbot, who is better known as Chainfire)