Sounds like you are in the Visual edit mode, not the Code edit mode.
From /***/wp-admin/post-new.php , click on Code tab (moving from the default Visual WYSIWYG mode, if there)
Cut and paste code from SlideShare ‘Embed in your blog’
Note: as noted by drmike above, not the ‘Embed (wordpress.com)’ code unless you are using WordPress for hosting.
Caveat: If you go back to Visual mode, (seen in WP 2.3.1) behind the scenes, WordPress messes with the code, typically moving the “/object” tag up in the code block
What I do, which saves considerable frustration:
1. Write the complete post without the embed code, leaving myself a placeholder note as in “[insert slideshare here]”
2. After all previews and any editing, go to Code tab if not already there, replace the placeholder note with cut-and-pasted code from SlideShare
3. Click on ‘Publish’ (never going into Visual mode)