Hi Thierno Barry ! Actually, if you want to implement Bayesian logistic regression, you do not need that normally distributed variable and the step function.
I strongly recommend you to see Page 19 from this tutorial: http://www.medicine.mcgill.ca/epidemiology/Joseph/courses/EPIB-668/bayesreg.pdf
Good luck with WinBUGS modelling, I believe it's a great tool. But you can use R instead, there will be more people who may help as R has wider community. HTH!
If you want to use a normally distributed variable (w), it seems you then cannot use the Bernulli-modelled variable at the same time. You need to use either Bernulli distribution or normal distribution. Otherwise the model cannot be correctly specified.
But what is the purpose of introducing this normally distributed variable?