Unfortunately the only way to do this is to use an FTP program to upload the folder “celestial” for the widgetkit plugin (slideshow) that is found in your original theme download package. IF you do not have an FTP program, you can download one (free) called FILEZILLA. Documentation is there too.
As a side note: Anyone running a website should have an FTP program because there are times where you may need to go into where all your files are where your website is. Web host providers have a file manager in the control panel, but these only allow you do upload one file at a time.
When you install Filezilla, you will have to create a site profile with the username, password, and ftp address (usually looks like this: http://ftp.yoursitedomain.com ) and then you can connect and see where your website files are. Luckily you only have to do this initial setup once.
The celestial style that you upload will go where the “default” folder is, which is the “styles” folder: