Numerous changes were made between 1.0.6 and 1.5.x with regards to custom cores. Somewhere I think there's a migration guide. But it may have been updated to 1.6.x, and while 1.5.x is a lot like 1.6.x, some things changed after it became 1.6.x...
Unfortunately, I don't remember what state the custom hardware definition system was in at 1.5.6 (several non-backwards-compatible changes were made in the 1.5.x and 1.6.x development cycle)
I recommend using 1.6.9, 1.6.5r5, or going back to the luddite-standard 1.0.6, rather than wasting time figuring out how to use an old, orphaned version of the IDE.
In 1.6.9 or 1.6.5r5, all you have to do to install an ATTiny core is paste the URL from my sig into preferences , then you can install it from tools -> boards -> board manager without any manual file shuffling.