I just downloaded the com_multisite_V1.2.21 file. When I try to install I get the following error message:

Fatal error: Maximum execution time of 30 seconds exceeded in /data/17/1/111/98/1600913/user/1725058/htdocs/joomla1/libraries/joomla/filesystem/archive/zip.php on line 304

Any ideas, please help!

JMS is an is big extension that makes more than 2Mb.
The error message appears because you have a timeout during the upload of the package.
If you have full control of you server, increase the timeout parameter in the "php.ini" file.

Otherwise, use unzip the package into a directory of your server and after use the "install from a directory" in joomla.
This will avoid you the timeout expired as the files are already uploaded and unzipped in a temporary directory.

Thank you. I finally got it to install, I had to use the Install From Directory method.

I am now having trouble installing the slave site. I am a newbie (in case you haven't already figured it out What I am trying to do is to have 2 sites share the same database but have totally different templates and act like separate sites, basic stuff right. I have both sites (really one site with 2 templates) up and running fine. I installed Multisite, started the setup and chose to install a Fresh Slave Site. When I did this it took me into the new Joomla config setup screens. When I got to the page asking for the database info I put in the info for the existing database and it is not working. Now I can't back out and can't get back to the Admin where I started setting things up. Maybe I am just not understanding how this works but I thought I am supposed to enter the database info for the current database, right?

I suggest that you start follow the tutorial to lean how JMS is working.
It shows how to create your first slave site into a subdirectory and later it shows how to share the users between website.
The sharing of extension require that you have MySQL user privileges.

So start to lean by creating a fresh slave site as shown in the tutorial. (ie. video 9, CP-03, CP04)
After that try replicate an existing website and for example see how to have specific image and templates folder (see video CP-05).

When you have understood how to do that in a subdirectory, you can try using the sharing.
See the video 16.
www.jms2win.com/tutorial

To be a little more specific, I am following the User Manual. When going through the Install Fresh Slave Site process here is the error message that I am getting:

Access denied for user '7_02b8416_0'@'%' to database 'joomla1' SQL=CREATE DATABASE `joomla1` CHARACTER SET `utf8`

I can access this database through phpMyAdmin with the same user/pass info and when I check on the host site it's the same login info as well. Just didn't know what the error was.

If I need to take a step back and start over that's fine too I would just need to know how to get out of this install process and be able to access my Joomla administrator backend again. Are there certain files that I can delete to reset things? The installation folder?

I was able to reset the configuration file. I think my host has some access restrictions. I will try again from the beginning. Thanks for your help.