river past help

I have got the miniusf files and usf files to convert to wav just fine. The problem I have is converting the wav to mp3. I am using River Past Wave@MP3 1.5.3 to convert the wav to mp3, but when I try and convert an error message says:

“Failed to connect DirectShow components. This is usually caused by bad file path, or absent or outdated DirectX installation.”

I do have a recent enough version of DirectX. It said I did when I was going through the set up process. I know I could use another program, but this one leaves the mp3 files a lot smaller than other wav to mp3 programs. Anyone come across this before? Or know how to fix this?

Is there a plugin I could use which just converts from usf to mp3? If so, would it compress the mp3 file as well as river past?