A tile on Symbaloo is a bookmark for a link. It takes a URL from a website and saves it to Symbaloo. That way you can access all of your websites from any place that you have internet connection. There is no way to access your computer files from any where other than your computer (in an ideal world) unless you upload them to the internet. Because of this, you cannot link a document, spreadsheet, picture, etc. straight from your computer to your Symbaloo webmix. You will have to upload it to the internet via Google Drive, Vimeo, Flickr, etc. This will give you a link that you can link your tiles to.
If you are wondering why Symbaloo does not offer a platform to upload your files to, this is because uploading files to a server can present the opportunity for malware to be uploaded. By not offering this service, we are trying to make Symbaloo safer for you to use.