I want to add a subscription block to my Drupal site. I found these two module: subscription and simple-subscription. I installed subscription (the first one) and put its related block in blocks section to right sidebar region but no block is shown.
Is this module reliable and where can i find a documentation for this module?
My guess would be permission problems.
If you are not logged in as user 1 and don't change default permissions, it probably won't show up.
In answer to your second question, one thing to determine if a module is reliable is to see how many other sites use it and how actively it is being worked on. If you see quite a few sites using the module and recent responses in the issues queue, then there is a good chance that it is a quality module or at the very least, being developed into a quality module.
I haven't used subscriptions, but it looks like it is maintained by quality developers and is used in many places. Here is some documentation http://drupal.org/node/344030
