I'm looking at downloading the C# binary for Bouncy Castle and see two downloads with source code. One has the IDEA algorithm, the others presumably not.
Why make separate distributions?
In some countries, IDEA is patented and redistributing an implementation of it requires it to be licensed. IDEA was a default cipher used in early versions of PGP. For backward compatibility, it is included in OpenPGP which Bouncy Castle implements.
For your case (a new application), get the distribution without it and use AES instead.