In order to make use of joins you will need to configure the join (left / right, which column(s) to join with etc). Usually this is just the patient identifier in the custom data and the patient identifier column in the dataset
Steps
For this example I have preloaded another table to act as a lookup, this will be added to the Biochemistry dataset
Step 1- Right click on the catalogue, Select 'Add' the 'Add a new lookup table relationship' 
Step 2- A new window will appear. In the box at the top select the file to join from the drop down list. The columns will appear below it. The linking item needs to be dragged down to the first primary key box on the right.
Step 3- The catalogue that is being joined to features on the bottom left the joining column (in this case 'ReadCode') needs to be dragged to the far right 'Foreign Key' box.
Step 4 - From the top box (Lookup table) drag the description column down to the large box on the right hand side marked as 'Descriptions'. 
Step 5 - The 'Create Lookup' box will change from grey to blank and the smiley face will appear. Select this and the description column will appear within Biochemistry
Step 6 – To finish off, you will notice the new descriptive field is marked 'yellow' for supplemental, this means that it will not automatically appear to extract. Right click on the column and select 'Set Extraction category'
Step 7 - From the pop up box select the 'Extraction Category', in our case we want this as 'Core' Then select 'ok' 
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