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First of all spend few minutes to find this property. Frequently property may exist but with a different name, e.g. logKow or Octanol/water partitioning coefficient is named as logPow in ochem. Just try to search for it using synonymous words. If you found it, there is no need to create it again. If property does not exist, you can easily create it. Go to browser of Properties and click "create new property". After this select its type: Numeric for properties which are numeric, e.g. have float values or Classification for properties like AMES, which could be "active" or "inactive" mutagens. For Numeric property correctly select "System of units". If your data are concentrations, select "Concentration". For properties like logPow, which are just ratios or logarithm of ratios, select default "Dimensionless". This is very important since will allow you to upload data, e.g. in mg/kg but develop models in log(mol/l): OCHEM provides an automatic conversion of units. For classification property add available classes, e.g. "active" or "inactive". Options for classification property should not be float/integer values, e.g. 0/1 will not be allowed by OCHEM. Finally provide some brief description of the property.
Download Ames training or test sets from http://ochem.eu/article/4027 to see an example how to prepare data for Classification properties.