What do I need in order to make my arduino to transmit / recieve signals exactly at 433.68 MHZ?
The "433MHz ISM Band" extends from 433.05 MHz to 434.79 MHz. (ISM = "Industrial Scientific and Medical".)  "433MHz" transmitters and receivers are programmed for whatever specific frequency you want to transmit (or receive) on.

For example, the very popular HopeRF product: RFM22B can be programmed to operate from 413 to 453 MHz. (Well below and above the official band.)  See the referenced data sheet, page 5. This is just one example of most "433MHz" products.

To talk to your 433.68 MHZ generator, you would simply use your Arduino code to tell your RF module to transmit at 433.68 MHz.


Ref: https://en.wikipedia.org/wiki/ISM_band 
Ref: https://www.sparkfun.com/products/10153 
Ref: https://www.sparkfun.com/datasheets/Wireless/General/RFM22B.pdf