1. It should be "models/player/frog/frog.mdl" ...If it didn't have an 's' it would mean a coding error...
2. Did you actually check to see if you really don't have that file?
3. If you don't have the file, then you must have missed the 3rd patch (18th Feb 07) which contained that file.
How to fix?
Simple easy way: Download the full version of the latest release (9th April 07)
Less downloading way: Download just the 3rd patch, then then run that patch and all the newer patches (again) in order.
Here's a list of each "Open Beta" release (R)
R1 - 5th Jan 07 --- no patch for ESF B1.2.3 -> ESF B1.3 OB-R1
R2 -20th Jan 07 --- 1st patch (R1 -> R2)
R3 - 4th Feb 07 --- 2nd patch (R2 -> R3)
R4 -18th Feb 07 --- 3rd patch (R3 -> R4)
R5 -18th Mar 07 --- 4th patch (R4 -> R5)
R6 - 9th Apr 07 --- 5th patch (R5 -> R6)