This depends on if you are using the "New Reaction Model..." wizard to create your reactions or if you create them by drag&drop/manually.

Wizard
If you are using the wizard you cannot choose type when creating the reaction, however you can do that once you have created it. To do this double-click on the newly created reaction in the class browser. Then right-click on the variable name in the Components view (found to the right of the diagram layer) and select  "Change type...". You can select type from:

* Builtin (Real, Integer, String, Boolean)
* SI units (as defined in Modelica.SIunits)
* Non-SI units (as defined in Modelica.Conversions.NonSIunits)
* or Other (where you can specify any unit the user has themselves defined).

Drag&Drop 
When you are adding a parameter to the model using the Insert->Parameter menu (on right-click in General tab), you can select the type from
the list above.

Text view
If you are adding the parameter in text-view you can write any type you want. Note that you can drag the type from the Class Browser into the text view to get it's fully qualified name (e..g BioChem.Units.AmountOfSubstance).