The Redeem voucher option in your Skype account can only be used to redeem Skype branded vouchers.
Ukash vouchers are third party, multi-purpose vouchers that can be used to purchase goods and services on many e-commerce websites.
You can use Ukash vouchers for Skype purchases in a selected number of countries. To find out if paying with Ukash is supported in your country, please visit the ways to pay page.
If purchasing with Ukash is supported where you are, then using your voucher is simple.