Work with streams in structured storage
Compound Storage format, used internally in MS Office binary files, is similar to the filesystem. It contains substorages and entries (like directories and files on the filesystem).
First you need to access the storage to get an TElCompoundFileStorage object. Then use TElCompoundFileStorage.RootEntry property to get the reference to the root entry, which is of TElCompoundFileStorageEntry type.
To read and write data from/to the stream see this how-to article.