just download source file from it.Then untar or unzip the file and put it in your plugins directory.Then in your layout include this line at the top
<%= calendar_date_select_includes "silver" %>.
you can choose silver,red,plain,blue also.
for your datefield you can specfiy the pattern whick u needed.
For my requirement i have given like this.
<%= f.calendar_date_select :Dateofbirth ,:time => false,:year_range =>60%>
For more details you can get from http://code.google.com/p/calendardateselect/