pragan - the error can be caused by a number of different jquery files issues. The best way to test is to open up the calendar page in your browser and the "view source" - this will show you the html from the calendar page. Then search for jquery.
We look for a few things
1) That the version of Jquery is higher than 1.7.2 - this is required by calpress
2) That the jquery file does not show up more than once - this sometimes happens when the theme loads a version, wordpress loads a version, and maybe a plugin. All of these can create conflicts.
If you want you can provide your url and we can take a quick look.
Also if you want you can submit a ticket on our zendesk site (calpress.zendesk.com) and that way can keep your infomation between you and us and not on a public board.