The help states

If you want to remove one or more items from the ignore list, right click on those items and select TortoiseSVN → Remove from Ignore List You can also access a folder's svn:ignore property directly. That allows you to specify more general patterns using filename globbing, described in the section below. Read Section 4.17, “Project Settings” for more information on setting properties directly. Please be aware that each ignore pattern has to be placed on a separate line. Separating them by spaces does not work.

See chapter 4.13 of the help for more details.

"Ignoring Files And Directories" from the Tortoise SVN documentation states:

If you want to remove one or more items from the ignore list, right click on those items and select TortoiseSVN → Remove from Ignore List You can also access a folder's svn:ignore property directly. That allows you to specify more general patterns using filename globbing, described in the section below. Read the section called “Project Settings” for more information on setting properties directly. Please be aware that each ignore pattern has to be placed on a separate line. Separating them by spaces does not work.