You can embed Slideshare slides using HTML & Iframe Macro for Confluence
Once you have installed the add-on, you can simply copy embed code from Slideshare which you can find by clicking share button below the presentation.
Open the Macro browser and add HTML/Iframe macro to the page you want to embed. Paste the copied embed code into macro body, save and publish the page. Slideshare presentation should appear in your page.
For example, I am embedding this presentation from Slideshare below: https://www.slideshare.net/svenpeters/software-for-every-team