Clear cache and redownload from the official site or download from my site
here (on download choose Local) then try installing again.
Also you need to be on an account with admin priviledges to install.
P.S. I decided to permanently mirror Plus! (Plus!3 and Plus!Live) on my site...you can find it in the DB on files page...on download you will get the choice to get it from Main Server (Plus! site) or from Local (local is the zip file i host).
/ 
