Doubts in modelling the Arthur(1991) paper: Designing economic agents that act like human agent
Although I still don't understand what these "strengths" are all about, still, mechanically, step $4)$ is now clear.

At each point in time, there is a certain allocation of Strengths to actions, $\{S_i(t),\; i=1,2,...,A\}$. This allocation changes through time.  

The allocation is turned into a relative frequency distribution by dividing each $S_i(t)$ by their sum $C(t)$.

But then 

$$\sum_{i=1}^A \frac {S_i(t)}{C(t)} =1 $$

and (I presume), $S_i(t) \geq 0,\; \forall i$. So indeed this relative frequency distribution can be treated as a probability distribution, and each $S_i(t)/C(t)$ can be treated as a probability that characterizes action $i$ in period $t$.  

So you have to code this probability distribution, and then have the software "randomly draw" from it, as though this distribution was part of  the distributions available in a "random number generator".  

The floor is yours now.