Select "Text" mode from the toolbar. The editor options dialog will open that additionally prompts for the text string. Don't forget to click "Apply" to take over the current string. If the dialog has been closed unintentionally, it can be reopened with the F3 shortcut.
To actually draw the text, move the mouse to the desired location and left-click to place it.
A text can be given a size which is stored in a GDS2 file (OASIS files do not provide this feature). The size of the text is only shown in the layout if a scalable text font is selected (the "Default" font is not scalable) and text scaling is enabled. In order to do so, choose a scalable font from the "Text font" selection box in the "Display" tab of the setup dialog and check the "Apply text scaling and rotation" box in the same tab.
The text can also be rotated, which is shown as well only if text scaling and rotation is enabled. To rotate a text while placing it, click the right mouse button. This will rotate the text by 90 degree counterclockwise.