Find My Mac – Functionality Understanding without GPS

Here's a way to manually create the Recovery HD partition, but it seems complicated for a novice user.

If you don't feel sure about your self into doing this, then I suggest you to re-install Lion.

The problem is that that your iMac is not waking from sleep.

Several people have been affected by this and discussed this on discussions.apple.com in these two threads. Both threads have been opened recently and deal with the "Wake on network access" problem in Lion.

It's required for your Mac to wake up because it needs to build up a Wi-Fi connection. Your iMac disconnects from the Wi-Fi in sleep mode. But without a connection to a known Wi-Fi network, Find my Mac cannot locate your computer.

This is what the iCloud troubleshooting guide says:

Find My Mac can only locate your Mac if it is connected to an internet via a known Wi-Fi network. If your Mac is connected to the internet only by an ethernet cable, Find My Mac will not be able to locate your Mac.

Idea

Does your iMac support wake on Airport network access? Newer Macs can wake up on both Airport and Ethernet access. This is then called "wake on network access" instead of "wake on ethernet/airport access".

If you iMac supports waking on Airport access, disable your Ethernet port. Maybe this will force the iMac to stay connected to the Wi-Fi during sleep and the 'bug' will not occur then.