How do I send a file by Bluetooth from Android smartphone to an iMac running OS X Lion?
Provided you have done what Vlad said (enable the Bluetooth sharing) and paired the devices (which you seem to have done), I have absolutely no problem to send files via bluetooth using my Nexus-S. I have ICS 4.03 now, but I had 2.3.x (where X is 3, 4, 5 and 6 if I'm not mistaken). 

Once you have the devices Paired, if the Mac is in range and with BT enabled, the phone can send files because the Mac should be on the phone's list of paired devices. I recall that in 2.3.x sometimes it won't work, but retrying to turning on/off BT was usually enough. I don't use it a lot, but I just tried (after months of probably never touching bluetooth on either device) and I got this: 



Here's my preference pane: 



and Finally, this is my Phone's paired list (two are intentionally blurred), but you can see my Macbook Air:



Hope you can double-check your settings. 

For reference:


I have Firewall ON
I have Firewall Stealth ON.


Here's my Firewall Screen (app list blurred by me):