I know what I want to achieve but cannot get started.
Take it one small-step at a time and start with something that you can test by itself... For example, can you build something that drives the divert switch/relay with a 5V low-current signal? Then you can run the Blink Example to make sure the Arduino can change the switch-state under software control.
Solid state relays can be great because you can get varieties that can be driven by the Arduino. But, you have to be a little careful on the output/power side because AC solid-stage relays won't switch DC, etc. And, you they are almost always SPST so you need multiple solid-state relays to make a DPDT configuration, etc. And, they are more expensive than electro-mechanical relays.
To drive a "high current" relay from the Arduino, you'll need a driver circuit (to get the required coil voltage/current) or you can buy a relay board with relays and drivers already installed.
On the input-side, do you have a phototransistor circuit that can detect light? i.e A circuit that is 0V with light and 5V with no light? (It's usually easier to use a pull-up resistor and use "inverted logic" like that.) If that works, is it sensitive enough to detect the LED?
Once the light detector is working, you can connect it to the Arduino and run the [u]Digital Read Serial Example[/u] to make sure the Arduino can detect the LED.
Then you can start on the programming. The two most important concepts in programming are conditional execution (if statements, etc.) and loops (doing something over-and-over, usually until some condition is reached). Once you understand those things, you can begin to understand how to make a program that does useful things.
For example, you can make a [u]while() loop[/u] that counts pulses for one second. Then you can exit the loop and decide what to do with the count. (Note that you can have nested loops).
For timing, look at the [u]Blink without Delay Example[/u] and try to understand how it works.
Note that you can write C++ statements that don't make sense in math: For example Count = Count +1; will increment the value of the variable named Count and if you put it in a loop you have a counter. (That can also be written as Count++;)
BTW - There is a [u]pulseIn()[/u] function that may help with what you're doing.
You may run into "issues" with the LED pulse not being "clean" or it may actually be pulsing faster than you can see when it appears "on", but that's something to work-out later if it comes-along.