i'm sorry for this, i know it's too late, but i never tried this game until now and had the same problem (the game wouldn't install saying "This client can only be installed and used at the start of Silkroad-R Open Beta Test."
Not a single solution worked for me except this one, finally.
First, if you already don't have the game, download it from here (it's the latest version (saying this in 18th of march 2013) of the game from Joymax's site): http://www.gamershell.com/download_97879.shtml
or here: http://www.ausgamers.com/files/details/html/70499
Once it downloads, unpack it, and then don't (!) start SilkroadOnline_SROROfficial_v1_065.exe, but right click on it, and choose "Extract to SilkroadOnline...", or you can right click on it and choose "Extract files" and then select the folder where you would want the game to be extracted (it doesn't matter where it is, but for both steps you will need WinRAR (it's a free file archiving program, which you'll need for this big "Silkroad blablabla.exe" file).
Now, once the game unpacks itself, go into that folder and open "Silkroad.exe". The game launcher should open, and will start updating and then allow you to finally play. If your firewall asks for permissions, allow it.
Every next time, you should open and play the game through this same "Silkroad.exe" (because if you click on other exes it will give you errors in-game).
i really hope this helps you as it helped me. Some got theirs to work after temporarily disabling their firewalls and antiviruses, but that never helped me anything so i found this other and only way that did, that i wrote now.