To register automatically, you can use the aescripts Manager app.
For manual registration run After Effects, create new composition, add a new solid layer, apply effect irrealix -> Volna or irrealix -> Reka grid and then click Register inside the Effects Control Window and enter your serial.
Note: If you still have red-X-watermark after entering serial number you need to clear cached frames. In After Effects menu: Edit -> Purge -> All memory & disk cache.