In the past I have successfully added engines for use in analysis in ChessBase 9. Crafty would be an example. These engines all ended up with a .eng extension in the engines folder.

Recently, I tried to add Rybka and Robbolito as UCI engines. The engines come in .exe form. I used the ChessBase create-a-UCI-engine feature. The conversion would seem to have gone well (a small UCI file was created that I presume would allow access to the .exe). However, each of these gave an error on trying to load. None of them loaded successfully.

What might I be doing incorrectly here? Any ideas?

I use Fritz what is almost the same as chessbase and it works ok. put Rybka*.exe and Robbolito*.exe in the enginefolder, chose create uci-engine from menu, ok, then change the engine in the menu

Prehaps go back and re-read the directions. Or, if you have followed all the directions, then contact their support

are there options when importing the engine, such as UCI or WINBOARD protocol?  i've had trouble with other interfaces in the past with UCI engines, but MOST of the time fritz/chessbase products do pretty well. 

Yes, ChessBase has a specific Create a UCI Engine option. It seems to create it OK, but when selected, I get an error message ... I'll have to try again to see what the message is, but something like "incompatible engine type" or some other such error.