The official website of this application is no longer in business, however, you have the possibility to download it through the websites listed on Google Search. As I can see, this is just a FTP client. There are plenty nowadays with advanced features and many other options. They're also free of charge. For example, FileZilla FTP Client is one of them. Download the client and use it just like you would have used Terrapin. There is no difference between them (maybe on the interface), but the base process is still the same.