Issue
If you have the situation that calendar is not visible in your booking form but all other fields is visible.

And you are have JavaScript error, similar to this:

Error: jQuery("#calendar_booking" + bk_type).datepick is not a function
Source: .../wp-content/plugins/booking/js/wpdev.bk.js
Line: 161

Reason
So possible reason of this issue is loading of jQuery library 2 times in your site/

The loading process is next:
1) Firstly is loaded the standard jQuery (ver 1.7) from wordpress (and its fine):

<script type='text/javascript' src='.../wp-includes/js/jquery/jquery.js?ver=1.7.1'></script>

2) Then load the scripts of the Booking Calendar, where is defined all functions, including the datepick funcion.
3) Then your theme (or may be some other plugin) is load the jQuery again, (its possible that its some old version of jQuery, like ver. 1.5).

<script type="text/javascript" src="..../wp-content/themes/my_custom_theme/js/jquery.min.js"></script>

This load of the jQuery is overwrite previous load and we are get that error.

Solution.
Please open the header.php file of your theme and deactivate (comment), the loading of the jQuery, so you do not load it for the second time.