Enter the pinyin of the word you see. Enter tone number (1,2,3 or 4) after the letter where the tone mark should appear. For example, type:

a1nji4ng for ānjìng

The number should turn into tone mark (slight delay possible).

For a, o, e, i and u, type tone number after letter, for example:

ā = a1, á = a2, ǎ = a3, à = a4

For ǘ, type the letter V.
ǖ = v1, ǘ = v2, ǚ = v3, ǜ = v4