to insert a model into a CS (or any HL based mod) map:
step 1. look for tutorials
step 2. download hammer 3.5 and the fgd that supports viewing models in hammer itself.
resources here - hammer 3.5 and here -fgd
step 3. after setting it all up (drag and drop hammer 3.5.exe over hammer 3.4.exe and creating a new Game Configuration using the new fgd - set it up for steam, as 1.5 and < don't support models like 1.6 < does), you place a point based entity in your map, select a cycler, open it's properties and browse teh model line, you need to select a model from your C:/steam/steam
apps/your@email.com/counter strike/cstrike/models folder, if you don't have any in there, download some from the doenerking's site, or browse Google.
Once you have selected your model you will be taken back to teh cycler's properties screen, double click teh model line, and copy the model's path (ie: models/tree.mdl).
Now pull down the drop-down list, and select cycler_sprite. By drop-down list, I mean the little tab above the entities properties list, where you chose cycler from at the begining.
Now in the cycler_sprite's properties paste the model's path into the sprite line.
Use the Black Circle with a White line as a radius to adjust teh model's direction (teh top down view corrallates to teh white line).
Now close teh cycler_sprite's properties and you should be looking at a wonderful model in your map.
If it crashes as you close it, then you need to create a new Game Configuration from Hammer eg: Tools/Options../Game Configurations.
And select only the 3.5 fgd you downloaded earlier.
If you can't see the model, it's just a small purple cube, you need to create a new Game Configuration as above.
The reason we use a cycler first is so that we can copy teh model's path exactly, don't try to type it in manually, as you will eventually make a mistake.
Remember model's can only be used from the mod's directory and nowhere else on your pc.
When releasing your map you must include teh model's like you would include sounds etc.