Get the unlocking service from your network provider ..Otherwise you can get it from online providers like Ihav.net .They offer the unlocking service for your mobile..Through this you can unlock your mobile easily..
If you want to unlock your Motorola Sidekick Slide mobile phone from the network lock,I would suggest you to get an unlock code from Superunlockcodes.com at reasonable price.After unlocking,you can use it with your service provider.