Para añadir a tu blog uno de estos botones de Google plus +1 tiene que ir a :
Si usas la interfaz antigua entra en Diseño | Elementos de la página | Añadir gadget | HTML/Javascript.
Si usas la nueva interfaz entra en Diseño | Añadir gadget | HTML/Javascript. y pega el codigo del boton que tu quieras.
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<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjoIyIgtMySzH1zHiBn-DVKy2A1_HQ0cQJBEstmHwdftu9B2zNDmuIHm6Q26LztkVlTrHiFvtSNJtwBEdPoGVAUAvEQ0in_0heaHojVqte7T5ARcowAptHkKtFrMK6ahyl_H_l7riAnthU/s100/google2.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg4BoNUNhI8VY3s_ET7TopT3QeHrK_Wn5mtQgAs4IIynQ58u48uLjMQZSyUNKxMjNFBM6p3IsP7HI37K6izbE37A1XJMSNDP2HeZyxcQUNHZqWGo82D5mSWHRHI_JeJ1kpEziuAPvZy0pk/s100/google3.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg0ZG-NtI19SWWl3r5X9_souk5tOFRQFHTMKbc5FNnLITMBObBVPoss3h7YEl9cv3BBjkLhbhyphenhyphenIqJcvxvitmtA9-y7FmcfwXAiIgHnSMXGGfvp61sV6bd5zO6F7A2agETldT4owFFCgV_8/s100/google4.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEip8Jv7vjNPRlkby0s2MaKnw-jVAiIKPxvghxyt8iPLc8fALfOnWFyka9iV8_L0eyQDNV99B7Qy2XnKEwW6sKuuDSlgEsCH1GRd9L4_xXV5ABV2S1bo7nq2dI6lcN2jtya3poFUMA-MtB8/s100/google5.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEirDX8o8k6ODRopl0TjWmV37ql1I9IrkuWql4iTL6VyRUTbaFTGrfdg1MtnZewaPZCAc6RSSKfRctta0us8J-FtHCrf0Mi5aFrZHx43ElMGqJ2kEL96tcuC6WEq9Pwr7c6BluMvRo9hk_Q/s100/google6.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjyRFBgoQOZSJo61KVFfRtbICpZ67VqrcF8x-dVTLrph9QbV7DEWsItaeVeaWUqho03f_TKq8Fz1xSKunNpKxphr9atxJtkenF7OkmfKWYg8JH16ZgpQlyTBA_mHqNLagWH4WaGzKpGoEE/s100/google7.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjyRFBgoQOZSJo61KVFfRtbICpZ67VqrcF8x-dVTLrph9QbV7DEWsItaeVeaWUqho03f_TKq8Fz1xSKunNpKxphr9atxJtkenF7OkmfKWYg8JH16ZgpQlyTBA_mHqNLagWH4WaGzKpGoEE/s100/google8.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiyF3wbhFUtuawbkBFypT63fjyoE_d-bZT2ZAaNECApjv9kmJTgjU54SsbpB3zbiVtfZAas__NZHYSt3FUMxGwfhTq5sEAz1tsSJbT-dMteOD3vucIawffUisILM-KZmzKbSJLK2fNhNsg/s100/google9.png" /></a>
<a style="" href="URL DE TU GOOGLE+"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOOexjKvuiLGGPJl9E05Zs5-QboEtJJd6lr7kR0BheRjqd8s-DBqvupv0ZLIe3Ayjdjwr6X7t6YjVGRvPA1AESUvNZZt8QgBPA63hQbZh3h2QQ-h1bpe0CJEmlPQA75-VGbtxqXqwTvK4/s100/google10.png" /></a>