The user synchronization plugin is published by default when hikashop is installed. The only two reasons it would be unpbulished would be:
you unpublished it yourself
the install process crashed in the middle before being able to publish the plugin (it could be a time out or memory limit because of restrictions on your hosting plan)
There are no reasons other than that.
For your second problem: paypal check that for your paypal account, each order number is unique. If you already used another e-commerce solution before using HikaShop, the id sent you paypal (the order ID in the orders listing) might already have been used by your previous ecommerce solution. The solution is to increase the increase in the database above the maximum order id of your previous software. For example, if you had 5000 orders. Set the auto_increment value of the hikashop_order table to 5001. Here is an example of how to do with phpmyadmin:
www.electrictoolbox.com/reset-auto-increment-value-mysql/