there can be 2 problems that i know of
i will discuss the one that i know how to fix first (its a bit confusing so read it carefully)
problem 1: you hae put the map in your minecraft saves folder and it will not show up on your map list, this is an easy fix
step 1: go in your minecraft saves folder
step 2: go into your downloads and find the ZIP file that the map is in
step 3: open the ZIP file (right click, open) if it says you cant open then right click, open with WINrar, IZarc or whatever archiver you have
step 4: once you have the zip file opened, double click on the folder and find the level.dat (there maybe files called old level.dat but just highlight all of the files that look like a peice of paper)
step 5: once you hilighted all of the files that look like a peice of paper, shrink the window that the minecraft saves folder is in (you know the button next to the X in the top right of the window) also shrink the ZIP file of the map then double click on the name of the map folder in the minecraft saves window
step 6: in the minecraft map folder, right click AND HOLD any of the hilighted files and frag them into the with space in the minecraft map folder (MAKE SURE you do not drag them into any of the files that looks like a folder)
problem 2: you downloaded and installed the map correctly but for some reason your not in the map your supposed to be in
problem 3: (in know theres only 2 but TOO BAD!) you only downloaded it and left it like that
step 1: go to your startup menu (if you have windows click the little color thing in the bottom left of the screen)
step 2: in the search bar type in "%appdata%" (without the "'s IMPORTANT: type in the percent signs to)
step 3: press the ENTER key
step 4: double click the .minecraft folder
step 5: double click the saves folder
step 6: go to your downloads and find the map's ZIP file
step 7: open the ZIP file
step 8: in the ZIP file of the map, right click and hold then drag the folder IN THE WHITE SPACE in the saves folder
(if you don't see the world in your minecraft maps while in the game then read "problem 1" at the top of this answer)
PLEASE READ THIS CUZ IT TOOK A LONG TIME TO TYPE!!!!!
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it wont you need to download the server thing of the minecraft website and replace the old one with it.
only minecraft classic multiplayer is free, beta mp is for premium only a.k.a buy the game.
either you don't have An internet connection or you have a cracked account on minecraft and it wont let you on. The alternative to this is their minecraft server was having issues.
probably not,unless u have friends that know ur server but strangers probably wont join ur game
its not out for 1.8 and it wont be worth it cuz 1.9 is out