I'm trying to create a visual map in QGIS 2.4 of the approximate distribution of journey origin points using a shapefile of UK postcode sectors, layer joined to a table with a column giving the number of journey origins in each sector. Using the Vector > Research Tools > Random Points menu tool, I can select the Input boundary layer and click the 'Use value from input field' radio button, but the pull-down menu to select the required field does not seem to work. If I then specify the Output Shapefile name and target location , I get an error message - "Errors: The following features IDs couldn't be calculated due to conversion error: 335, 335". To confirm, QGIS does display random points if I specify the density or a specific number of points using the first two 'Stratified Sampling' Designs for individual polygs.
When using the Random Points tool (from the toolbar: Vector > Research Tools > Random Points), the "Use value from input field" only accepts integer type fields.
As @PeterA suggested, field types can be seen from Layer Properties > Fields which can be used to create new fields and/or access the Field Calculator for additional functionality.