// *** In your class header:

@interface MyInputController : IMKInputController {

    // ...
    // The rest of your input controller definition, omitted from this snippet.
    // ....

    // Dictionary that maps candidate string to annotation string for candidates window
    NSDictionary *annotationDictionary;

}

@end

// *** In your class implementation:

- (NSArray*)candidates:(id)sender
{
    NSMutableArray *theCandidates = [NSMutableArray array];

    // ...
    // Build theCandidates array of candidate NSStrings from converted text input. This depends on 
    // how you implement your IM and is beyond the scope of this code snippet.
    // ...

    if (annotationDictionary == nil) {
        // Create NSMutableArray of NSAttributedStrings based on our candidate strings
        NSMutableArray *annotationStrings = [NSMutableArray array]; 
        for (NSString *candidateString in theCandidates) {
            // Determine an appropriate annotation string for the candidate. Again, how this
            // is determined is up to your IM to implement, and is beyond the scope of this snippet.
            [annotationStrings addObject:[self annotationForCandidateString:candidateString]];
        }
        // Create the instance-owned dictionary that maps candidate strings to annotations
        annotationDictionary = [NSDictionary dictionaryWithObjects:annotationStrings forKeys:theCandidates];
    }

    return theCandidates; 
}

- (void)candidateSelectionChanged:(NSAttributedString*)candidateString
{
    // ...
    // Change marked text for candidate as necessary
    // ...

    // Determine if the selected candidate string has an annotation. It is up to your IM to
    // come up with this. If we continue from the first code snippet, we can use the 
    // annotationDictionary to look up an annotationString.

    extern IMKCandidates *candidates;

    NSAttributedString *annotationString = [annotationDictionary objectForKey:candidateString];
    if (annotationString != nil) {
        [candidates showAnnotation:annotationString]; 
    }
}