You need to add them to your worlds on the create / load world menus. there is a buttom for mods, and it will display all your subscribed mods and you can choose which to use on each world.

i went to new world and pressed custom and pressed mods AND IT WASNT THERE

if it is not in the mod list after you subscribe i am not sure what is goping on.

Check to be sure you are subscribed, and check to see if SE is set to auto update. it may not DL mods if it is not set for auto update. Also my try unsubscribing and then re subscribe.

As all you need to do is hit the subscribe button and the mod will be automatically installed for you.