WORKAROUND
Check Compression together with the option Use XML Specification.
1.Open a model
2.R-click | Edit to Edit entity
3.Go to "Indexes" tab
4.Select one index
5.Click on Edit
6.Go to "index Properties" tab
7.check Compression and Use XML Specification checkbox
8.Click Apply or Ok.
STATUS
Waiting for fix in a future release of Toad Data Modeler