Can you please translate this, 'm pa janm konnen ou te ka alsiyis konsa', for which online translating tools is no help. Thanks

Oh man! That is some serious r-rated stuff :)
M pa janm konnen ou te ka alsiyis konsa - I didn't know you could moan like that.
(sexually related)
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words

What is alsiyis?

Alsiyis is moaning (sexual). Basically it is 'sex noises'.
That question was already answered here before. Here is the link for usage: alsiyis

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words

How exactly is the word 'alsiyis' used?

Alsiyis (to moan in pleasure, or moans of pleasure) can be used as verb or a noun.
egz:
Msye fè m alsiyis nèt. 
Se pa ti Alsiyis fanm nan bay non!
 
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words