Q: HOW CAN INSTALL OS X LEOPARD FROM DVD EXTERNAL

PowerPC Macs typically only start up from external drives, if they are FireWire (not USB).  What's wrong with the PowerBook's internal optical drive?

That "DVD" is a Mac OS X installation disc?  As a test, insert another type of disc into the internal optical drive.  For example, what happens if you insert a music CD?  Or a movie DVD?  Do they also get ejected without mounting?

If your disk does not look like:

Or it says update, dropin or OEM on it, it will not work on your Mac.    Note, installing Leopard on the said Mac will disable Classic, the old operating system that came with it.  

'C' key booting may sometimes fail to work.  See if using the Startup Manager helps, and only install after backing up your data*:

http://www.macmaps.com/backup.html

See this tip more on installing Leopard:

How to Install Leopard Troublefree

* Links to my pages may give me compensation.

Sorry, that's a Machine specif disc & Intel only, no way it'll work on a PPC Powerbook.

Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...

http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5

You're welcome, & good luck!