There are two ways to search and find content in Merlin Project. One is by Find, another by using Filters. When to use one, or the other?
Find searches the entered string and highlights it on visible columns. You may use search patterns or enter a string to be replaced with. So if you want to search for (and replace) some text in your notes, you need to enable the 'Notes' column first, or switch to the pre configured view 'Work breakdown > Notes' which contains your text columns.
Another option would be to filter the view. You can create a new filter, select in its criteria the 'Notes' column and enter the string you are looking for. So if you want to filter the rows no matter if the column 'Notes' is visible or not, you enable a filter.