I am not an expert. I found this explanation on the internet and it worked for me, however I suspect my laptop an i3 processor with 4MG Ram may not powerful enough to run Houdini with 4CPUs because it runs at 100%. So I have to lower it to 2 CPU. It is important to know that Houdini is a Chess engine, and like a car you can't drive it without a body, Fritz is a GUI, Graphics User Interface, this is the body of the car. So Houdini has to be installed inside Fritz before it can run. Anyway, this is how I installed it on Fritz. 1. Download Houdini and note were it is saved. It is a Zipped File so Extract the files and note were they are saved. 3. In Fritz select Engine the select Create UCI engine. 4. Browse to find the downloads and select the engine, 64 bit or 32 bit, the 64 bit is stronger but needs a 64 bit processor. 5. Houdini will appear in the Name box. 6. Select OK. Houdini will now be an Engine in Fritz. 7. In the Engine menu Select Change Main Engine and Select Houdini. That is it. Having installed Houdini it is interesting to play it against Ftitz. To do this select Engine Match. Note. If you have a 64 bit processor you can download both engines, if not just download the 32 bit version.
Hope this helps.