Welcome to the forum.
It could be that your RAM is not properly supported in the UEFI on that board.
AsRock is correct when they say memory frequencies of upto 2800MHz are supported, but that is only with a 3rd generation Core i5 such as 3570K, 2nd Gen Core i5 such as the 2500K will normally only be able to reach 2133MHz.
Both Sandy Bridge and Ivy Bridge CPUs have an integrated memory controller. The Ivy Bridge controller is both faster and more flexible.
You should be able to select 2133MHz. from the OC Tweaker page/ DRAM frequency, as in the screenshot below.
This is on an AsRock Z68 Extreme 4 board with a 2600K.
Make sure the base clock frequency is set at 100MHz, and disable "spread spectrum" to lock it at 100MHz.