There's a number of things that could be going on. Here's a couple of them. If you'd like to respond with more specifics I might be able to narrow down the cause.
You say you have Lion installer on an external HD. You need to be sure you are booting from a copy of the Recovery HD, not from a clone of bootable clone of a Lion installation. The Recovery HD is a hidden partition, and you'd have had to purposely created it either with Apple's Recovery Disk Assistant or by using something like Carbon Copy Cloner to clone your boot drive. If that all sounds like gibberish, then we've probably found your problem.
If you are trying to boot from a copy of the Recovery HD, we need to look at where it came from. If it was created from a mac that shipped with Lion pre installed, then it simply won't work on another mac. If it was created from a mac that had Snow Leopard and then was upgraded to Lion, then it should work on your MacBook Pro, assuming it has Snow Leopard installed. You can't upgrade from a previous version of OS X without upgrading to Snow Leopard first.