The demo is not meant to be used during your current campaign. My idea for offering it this way, before putting the files for sale, is that people can download and check how it works beforehand. In this way, a potential customer can influence the direction of the product. There could be little features not currently included which might be of use to some groups. If pointed out early enough, they could be included in the version that we will offer through RPGNow.
As regards using it on a tablet, the answer has 2 sides: the ERA Server must run on a Windows, Mac or Linux computer. Android and iOS are not suitable for the server. BUT once the server is running on a computer, the application interface can be accessed through any browser in the same WiFi network. That could be the same computer that is running the server, a tablet, smartphone, etc.
This is not the same as implying that it will be comfortable to use it from them. The character creation module is meant to be used in a screen of at least 10'', and I would even suggest 14'' (though a player in my group once leveled up using my 7'' tablet, and with some effort got the job done). The adventuring module should be useful from screens of 10'' up, and my recommendation is to do it with access to a keyboard. The character status module is specifically designed to be accessed by the players' smartphones, so any device of 4'' or more will be fine.