sharing other content from iMac via appletv

The video must be in iTunes and in a compatible format
http://www.apple.com/appletv/specs.html

The iMac must be on and running iTunes

Alternatively you could put the videos onto an ios device and use airplay

Can I share the folder on the iMac? Is there a way to do this?

Yes, but if, and only if, the files are already TV compatible. Basically, you simply "alias" the import using the "Option" key. This will add the files to the iTunes library database but not physically add them to the designated library storage device. I.e., the files become playable from their current physical location but are logically managed in iTunes if in an iTunes compatible file container and playable via TV if already in an TV compatible compression format.

I understand the alias concept, just not how to accomplish it.

Simply press/hold the "Alt/Option" key while "drop-importing" one or more files. iTunes will create a "reference" pointing to the file(s) for "logical" management purposes but not physically import the file to your designated library area. That is, the original file remains in its original physical location but for all intents and purposes, iTunes acts as if the file where physically imported to the library. Further, if you later decide to delete the file from the iTunes library, only the reference link will be deleted, leaving the file still available in its original physical location. This is a built-in iTunes feature.