You will have to insert the required column (MS Project doesn't have an "unhide column" option). Right-click on an existing column heading and choose "insert column" option. After this, select the column/field you want. The new column will appear next to the column you had chosen.
If you plan to temporarily hide a column, do not use the "hide column" option since you will lose the custom headings that you may have defined (the data remains intact of course). Instead, using your mouse, click the column boundary and move it from right to left till the column is hidden. To unhide, drag from left to right. Saving Changes...
