How to control a simulink model by means of XPC target library?

Hi everybody, I am asking a huge help, because I have to finish my thesis project and I received license for R2012b XPC Target library late and I have small time to finish everything. I hope your suggestions can help me.

I have developed a simulink model using Simpowersystems blocks and now I would like to control it by XPC Target. My model actually has a timer for sending interrupts but I am wondering if I can replace this block with another which allows to control the model from external way (with a real control panel or another computer where I run some script in place of Timer step). Could you explain if/how it's possible?and the procedure I should follow?

In the future the purpose is to replace even other kind of blocks ( e.g. switch) and integrate the external hardware like if it was really inside the model, capable to read support same input/output which belongs to the block it's replacing.

thank you for everything you will say. Bye, Chiara