how to design a power amplifier's layout in l-edit?
You seem to be going about things in a bit of an odd way, L-Edit is used for laying out IC's, from Tanner EDA you should really be using s-edit to design the schematic before laying out, as mistakes at the IC level can be very costly.

I believe that the .ext file is the extract definition file, the format for this is in the manual, it has been some time since I have looked at this.

Normally your silicon foundry will provide you with a process development kit which will include the design rules and extract definition. If not you will need to create this or get someone else to do this. I know tanner EDA and the European distributers EDA solutions provide this service but that costs money.

The Dev-gen function is only useful for creating individual devices such as capacitors and mosfets, to go from there you will need to create metal interconnects.